Plumb CompositingReason from Blink to compositor.

public/platform/WebLayer.h

Index: public/platform/WebLayer.h
diff --git a/public/platform/WebLayer.h b/public/platform/WebLayer.h
index 599e6d881777752613fb21fc9ccdfe3df9f5bdac..3cb381a11b503e5598eeaa3915fc37cb57416429 100644
--- a/public/platform/WebLayer.h
+++ b/public/platform/WebLayer.h
@@ -29,6 +29,7 @@
 #include "WebAnimation.h"
 #include "WebColor.h"
 #include "WebCommon.h"
+#include "WebCompositingReasons.h"
 
 // Remove after making setPositionConstraint() pure virtual.
 #include "WebLayerPositionConstraint.h"
@@ -140,6 +141,10 @@ public:
 
     virtual void setDebugName(WebString) = 0;
 
+    // Provides a bitfield that describe why this composited layer was created.
+    // FIXME: non-pure until the chromium-side implements this.
+    virtual void setCompositingReasons(WebCompositingReasons) { }
+
     // An animation delegate is notified when animations are started and
     // stopped. The WebLayer does not take ownership of the delegate, and it is
     // the responsibility of the client to reset the layer's delegate before