Add UMA metrics to OffscreenCanvas's commit type

third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.h

Index: third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.h
diff --git a/third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.h b/third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.h
index 16ffb5eed008ccc42af672f3314f1278af3ab5ee..d6353f9b4f47b0b4001e459a06caeb3208187bcd 100644
--- a/third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.h
+++ b/third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.h
@@ -37,6 +37,14 @@ class PLATFORM_EXPORT OffscreenCanvasFrameDispatcherImpl final
   void ReturnResources(
       Vector<cc::mojom::blink::ReturnedResourcePtr> resources) override;
 
+  enum OffscreenCanvasCommitType {

[Ilya Sherman, 2016/10/11 00:54:11]

Please document that this enum is used to back an UMA histogram, and should
therefore be treated as append-only.

[xidachen, 2016/10/11 17:21:16]

Done.

+    CommitGPUCanvasGPUCompositing = 0,
+    CommitGPUCanvasSoftwareCompositing = 1,
+    CommitSoftwareCanvasGPUCompositing = 2,
+    CommitSoftwareCanvasSoftwareCompositing = 3,
+    OffscreenCanvasCommitTypeCount,
+  };
+
  private:
   const cc::SurfaceId m_surfaceId;
   const int m_width;