Add field trial for enabling Stage3D in XP

content/public/common/content_constants.cc

Index: content/public/common/content_constants.cc
diff --git a/content/public/common/content_constants.cc b/content/public/common/content_constants.cc
index b8e205f9dc72926cab2e16cd5e66840bf42c19c6..91522954fc42bc61d37d1cdbfe1b63554ceb1578 100644
--- a/content/public/common/content_constants.cc
+++ b/content/public/common/content_constants.cc
@@ -39,4 +39,8 @@ const char kGpuCompositingFieldTrialName[] = "ForceCompositingMode";
 const char kGpuCompositingFieldTrialForceCompositingEnabledName[] = "enabled";
 const char kGpuCompositingFieldTrialThreadEnabledName[] = "thread";
 
+const char kStage3DFieldTrialName[] = "EnableStage3D";
+const char kStage3DFieldTrialBlacklistedName[] = "blacklisted";
+const char kStage3DFieldTrialEnabledName[] = "enabled";
+
 }  // namespace content