Fix a few latency related UMA calculation

1. When using input event's LatencyInfo to compute latency related UMA metric
(Event.Latency.RendererImpl.GestureScroll & Event.Latency.TouchToScrollUpdateSwap)
the current implementation counts the averaged latency component as just 1 sample.
This underestimates the latency component's contribution to the UMA metric when
the latency component's event_count is larger than 1.

The correct way is to add |event_count| samples to the UMA metric distribution.

2. When set the touch event's acked time as INPUT_EVENT_LATENCY_ACKED_COMPONENT,
current implementation loops through the uncoalesced touch event list and call
the HighResNow() for each touch event. But actually all the events are acked at
the same time as a whole coalesced event, they should have the same acked timestamp.

BUG=None.
TEST=1. On Pixel, do touchscreen scrolling on page  www.rbyers.net/janky-touch-scroll.html
     2. Check in chrome://histograms/ that
        average Event.Latency.TouchToScrollUpdateSwap
	>  average Event.Latency.RendererImpl
        >  average Event.Latency.Browser.TouchAcked

Review URL: https://chromiumcodereview.appspot.com/20155005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@214833 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 20 insertions, 14 deletions

diff --git a/content/browser/renderer_host/input/touch_event_queue.cc b/content/browser/renderer_host/input/touch_event_queue.cc
index 0a7b1d4..56500e6 100644
--- a/content/browser/renderer_host/input/touch_event_queue.cc
+++ b/content/browser/renderer_host/input/touch_event_queue.cc
@@ -188,12 +188,13 @@ void TouchEventQueue::PopTouchEventWithAck(InputEventAckState ack_result) {
   // to the renderer, or touch-events being queued.
   base::AutoReset<bool> dispatching_touch_ack(&dispatching_touch_ack_, true);
 
+  base::TimeTicks now = base::TimeTicks::HighResNow();
   for (WebTouchEventWithLatencyList::const_iterator iter = acked_event->begin(),
        end = acked_event->end();
        iter != end; ++iter) {
     ui::LatencyInfo* latency = const_cast<ui::LatencyInfo*>(&(iter->latency));
-    latency->AddLatencyNumber(
-        ui::INPUT_EVENT_LATENCY_ACKED_COMPONENT, 0, 0);
+    latency->AddLatencyNumberWithTimestamp(
+        ui::INPUT_EVENT_LATENCY_ACKED_COMPONENT, 0, 0, now, 1);
     client_->OnTouchEventAck((*iter), ack_result);
   }
 }
diff --git a/content/browser/renderer_host/render_widget_host_impl.cc b/content/browser/renderer_host/render_widget_host_impl.cc
index c08d897..858447c 100644
--- a/content/browser/renderer_host/render_widget_host_impl.cc
+++ b/content/browser/renderer_host/render_widget_host_impl.cc
@@ -2439,12 +2439,14 @@ void RenderWidgetHostImpl::FrameSwapped(const ui::LatencyInfo& latency_info) {
     // results in final frame swap.
     base::TimeDelta delta =
         latency_info.swap_timestamp - original_component.event_time;
-    UMA_HISTOGRAM_CUSTOM_COUNTS(
-        "Event.Latency.TouchToScrollUpdateSwap",
-        delta.InMicroseconds(),
-        0,
-        1000000,
-        100);
+    for (size_t i = 0; i < original_component.event_count; i++) {
+      UMA_HISTOGRAM_CUSTOM_COUNTS(
+          "Event.Latency.TouchToScrollUpdateSwap",
+          delta.InMicroseconds(),
+          0,
+          1000000,
+          100);
+    }
     rendering_stats_.scroll_update_count += original_component.event_count;
     rendering_stats_.total_scroll_update_latency +=
         original_component.event_count *
diff --git a/content/renderer/gpu/input_handler_proxy.cc b/content/renderer/gpu/input_handler_proxy.cc
index 74cc6d1..1d9bb07 100644
--- a/content/renderer/gpu/input_handler_proxy.cc
+++ b/content/renderer/gpu/input_handler_proxy.cc
@@ -36,12 +36,15 @@ void SendScrollLatencyUma(const WebInputEvent& event,
   if (it == latency_info.latency_components.end())
     return;
 
-  UMA_HISTOGRAM_CUSTOM_COUNTS(
-      "Event.Latency.RendererImpl.GestureScroll",
-      (base::TimeTicks::HighResNow() - it->second.event_time).InMicroseconds(),
-      0,
-      200000,
-      100);
+  base::TimeDelta delta = base::TimeTicks::HighResNow() - it->second.event_time;
+  for (size_t i = 0; i < it->second.event_count; ++i) {
+    UMA_HISTOGRAM_CUSTOM_COUNTS(
+        "Event.Latency.RendererImpl.GestureScroll",
+        delta.InMicroseconds(),
+        0,
+        200000,
+        100);
+  }
 }  // namespace
 
 }