Add UMA metric: Startup.BrowserMainToRendererMain.

This new metric keeps track of the time from BrowserMain() to
RendererMain().

Time is recorded by renderer processes when they enter RendererMain().
This time is sent to the browser through an IPC. The browser records the
first renderer main entry time that it receives and ignores the others.
When the first non-empty paint occurs, the browser computes
Startup.BrowserMainToRendererMain
(First Renderer Main Entry Time - Browser Main Entry Time) and logs it.

We need a renderer->browser IPC because the startup temperature is only
known in the browser (we cannot pass this information with a cmd-line
flag because it is not known yet when the first renderer is launched).

We need to wait until the first non-empty paint to log the metric,
because we don't have any guarantee that both the startup temperature
and the first renderer main entry time are known before that.

BUG=547794

Review URL: https://codereview.chromium.org/1413533008

Cr-Commit-Position: refs/heads/master@{#359904}

1 files changed, 1 insertions, 0 deletions

diff --git a/ipc/ipc_message_start.h b/ipc/ipc_message_start.h
index c151c547..82786b5 100644
--- a/ipc/ipc_message_start.h
+++ b/ipc/ipc_message_start.h
@@ -132,6 +132,7 @@ enum IPCMessageStart {
   ArcInstanceMsgStart,
   ArcInstanceHostMsgStart,
   DistillerMsgStart,
+  StartupMetricMsgStart,
   LastIPCMsgStart  // Must come last.
 };