Add field trial for enabling Stage3D in XP

chrome/browser/chrome_gpu_util.cc

Index: chrome/browser/chrome_gpu_util.cc
diff --git a/chrome/browser/chrome_gpu_util.cc b/chrome/browser/chrome_gpu_util.cc
index 90efc1246768600b631690ea153dc4a5f08a06d2..d5097665d16e6ca35b4e6ff6e791fdead06e2b58 100644
--- a/chrome/browser/chrome_gpu_util.cc
+++ b/chrome/browser/chrome_gpu_util.cc
@@ -122,6 +122,43 @@ void InitializeCompositingFieldTrial() {
   UMA_HISTOGRAM_BOOLEAN("GPU.InCompositorThreadFieldTrial", thread);
 }
 
+bool ShouldRunStage3DFieldTrial() {
+#if !defined(OS_WIN)
+  return false;
+#else
+  if (base::win::GetVersion() >= base::win::VERSION_VISTA)
+    return false;
+  return true;
+#endif
+}
+
+void InitializeStage3DFieldTrial() {
+  if (!ShouldRunStage3DFieldTrial()) {
+    base::FieldTrial* trial =
+        base::FieldTrialList::Find(content::kStage3DFieldTrialName);
+    if (trial)
+      trial->Disable();

[Alexei Svitkine (slow), 2012/09/19 20:33:16]

Disabling the trial has the effect of choosing the default group. I think that's
not what you want here since you've chosen your default to be "blacklisted". I'm
assuming you don't want to blacklist users on Windows 7.

I think you probably want to make "Enabled" your default group, then disabling
the trial on non-XP platforms will have the correct behavior. In this scenario,
you would add the "blacklisted" group with 100% when creating the trial (which
happens only under XP).

Then, Stage3DBlacklisted() call in gpu_data_manager_impl.cc could simply check
the state of the trial (in the way I suggest below) without needing to also do
platform checks there. It will work correctly because:

1. On XP with no trial config, it will use the client set up code which will add
"blacklisted" as 100%, checking for "blacklisted" always works.
2. On XP with trial config, checking for "blacklisted" will work per config
probabilities.
3. On non-XP without trial config, trial is not created, so group value will not
be "blacklisted", so all is good.
4. On non-XP with trial config, trial will be created by config and then
disabled by this code, taking the default group of "enabled".

("trial config" means what we get from the server.)

+    return;
+  }
+
+  const base::FieldTrial::Probability kDivisor = 1000;
+  scoped_refptr<base::FieldTrial> trial(
+    base::FieldTrialList::FactoryGetFieldTrial(
+        content::kStage3DFieldTrialName, kDivisor,
+        content::kStage3DFieldTrialBlacklistedName, 2013, 3, 1, NULL));
+
+  // Produce the same result on every run of this client.
+  trial->UseOneTimeRandomization();
+
+  // Kill-switch, so disabled unless we get info from server.
+  int enabled_group = trial->AppendGroup(
+      content::kStage3DFieldTrialEnabledName, 0);
+
+  bool enabled = (trial->group() == enabled_group);
+
+  UMA_HISTOGRAM_BOOLEAN("GPU.Stage3DFieldTrial", enabled);
+}
+
 // Load GPU Blacklist, collect preliminary gpu info, and compute preliminary
 // gpu feature flags.
 void InitializeGpuDataManager(const CommandLine& command_line) {