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 9b0cdc4fe98ecc99a11c4f90c1c3b6b98740f256..71ea99cce6d6a45915e581c414933a09a8f15bed 100644
--- a/content/public/common/content_constants.cc
+++ b/content/public/common/content_constants.cc
@@ -41,4 +41,8 @@ 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