Histograms & Field Trails about prefetching

This CL lands some histograms to track prefetching, and a 10% field trial
enabling the feature to track performance.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/3036050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@55431 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 23 insertions, 0 deletions

diff --git a/chrome/browser/browser_main.cc b/chrome/browser/browser_main.cc
index cf806fc..5f8fa20 100644
--- a/chrome/browser/browser_main.cc
+++ b/chrome/browser/browser_main.cc
@@ -171,6 +171,7 @@ void BrowserMainParts::EarlyInitialization() {
   SocketTimeoutFieldTrial();
   ProxyConnectionsFieldTrial();
   SpdyFieldTrial();
+  PrefetchFieldTrial();
   InitializeSSL();
 
   PostEarlyInitialization();
@@ -364,6 +365,28 @@ void BrowserMainParts::SpdyFieldTrial() {
   }
 }
 
+// If neither --enable-content-prefetch or --disable-content-prefetch
+// is set, users will be in an A/B test for prefetching.
+void BrowserMainParts::PrefetchFieldTrial() {
+  if (parsed_command_line().HasSwitch(switches::kEnableContentPrefetch))
+    ResourceDispatcherHost::set_is_prefetch_enabled(true);
+  else if (parsed_command_line().HasSwitch(switches::kDisableContentPrefetch)) {
+    ResourceDispatcherHost::set_is_prefetch_enabled(false);
+  } else {
+    const FieldTrial::Probability kPrefetchDivisor = 100;
+    const FieldTrial::Probability no_prefetch_probability = 90;
+    scoped_refptr<FieldTrial> trial =
+        new FieldTrial("Prefetch", kPrefetchDivisor);
+    trial->AppendGroup("ContentPrefetchDisabled", no_prefetch_probability);
+    const int yes_prefetch_grp =
+        trial->AppendGroup("ContentPrefetchEnabled",
+                           FieldTrial::kAllRemainingProbability);
+    const int trial_grp = trial->group();
+    ResourceDispatcherHost::set_is_prefetch_enabled(
+        trial_grp == yes_prefetch_grp);
+  }
+}
+
 // BrowserMainParts: |MainMessageLoopStart()| and related ----------------------
 
 void BrowserMainParts::MainMessageLoopStart() {