Adding a field trial for testing --force-compositing-mode on 50% of

chrome/browser/chrome_browser_main.cc

Index: chrome/browser/chrome_browser_main.cc
===================================================================
--- chrome/browser/chrome_browser_main.cc	(revision 140632)
+++ chrome/browser/chrome_browser_main.cc	(working copy)
@@ -160,6 +160,7 @@
 #if defined(OS_MACOSX)
 #include <Security/Security.h>
 
+#include "base/mac/mac_util.h"
 #include "base/mac/scoped_nsautorelease_pool.h"
 #include "chrome/browser/mac/keystone_glue.h"
 #endif
@@ -514,6 +515,10 @@
     "Unable to find locale data files. Please reinstall.";
 }  // namespace chrome_browser
 
+namespace chrome_browser_trials {
+bool g_in_force_compositing_mode_trial = false;

[jar (doing other things), 2012/06/07 16:10:44]

Rather than using a global, which then puts a need to include the header for
this file....  The field_trial.h defines the generic test function:
base::FieldTrialList::Find(...)
Which can be used to answer the question from anywhere (never a problem with
#include dependency issues).

*IF* you really find you need to often ask the question, you can certainly cache
the result locally, assuming you understand the initialization order.  In this
case, you're correctly calling for this to be set up before we even go
multi-threaded, so you are assured this will be done in advance of most any
uses.

+}  // namespace chrome_browser_trials
+
 // BrowserMainParts ------------------------------------------------------------
 
 // static
@@ -1011,6 +1016,42 @@
   }
 }
 
+void ChromeBrowserMainParts::ForceCompositingModeFieldTrial() {
+// Enable the field trial only on desktop OS's.
+#if !(defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_LINUX))
+  return;
+#endif
+  chrome::VersionInfo::Channel channel = chrome::VersionInfo::GetChannel();
+  // Only run the trial on the Canary channel.
+  if (channel != chrome::VersionInfo::CHANNEL_CANARY)
+    return;
+#if defined(OS_WIN)
+  // Don't run the trial on Windows XP.
+  if (base::win::GetVersion() < base::win::VERSION_VISTA)
+    return;
+#elif defined(OS_MACOSX)
+  // Accelerated compositing is only implemented on Mac OSX 10.6 or later.
+  if (base::mac::IsOSLeopardOrEarlier())
+    return;
+#endif
+
+  const base::FieldTrial::Probability kDivisor = 100;
+  scoped_refptr<base::FieldTrial> trial(
+    base::FieldTrialList::FactoryGetFieldTrial(
+        "ForceCompositingMode", kDivisor, "disable", 2012, 12, 31, NULL));
+
+  // Produce the same result on every run of this client.
+  trial->UseOneTimeRandomization();
+  // 50% probability of being in the enabled group.
+  const base::FieldTrial::Probability kEnableProbability = 50;
+  int enable_group = trial->AppendGroup("enable", kEnableProbability);
+
+  chrome_browser_trials::g_in_force_compositing_mode_trial =
+      (trial->group() == enable_group);
+  UMA_HISTOGRAM_BOOLEAN("GPU.InForceCompositingModeFieldTrial",
+      chrome_browser_trials::g_in_force_compositing_mode_trial);
+}
+
 void ChromeBrowserMainParts::DomainBoundCertsFieldTrial() {
   chrome::VersionInfo::Channel channel = chrome::VersionInfo::GetChannel();
   if (channel == chrome::VersionInfo::CHANNEL_CANARY) {
@@ -1102,6 +1143,7 @@
   DefaultAppsFieldTrial();
   AutoLaunchChromeFieldTrial();
   DomainBoundCertsFieldTrial();
+  ForceCompositingModeFieldTrial();

[jar (doing other things), 2012/06/07 16:10:44]

nit: I think you should call this a "CompositingMode" field trial.  When I read
the function name, it looks like "Force" the use of the "CompositingMode" field
trial.

   SetupUniformityFieldTrials();
   AutocompleteFieldTrial::Activate();
 }