Adding feature usage count for OffscreenCanvas

third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.idl

Index: third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.idl
diff --git a/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.idl b/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.idl
index 4625e717252ea25b62e081fd56175c5b6d69ce94..a378e20c7bdec0d9fa32fca015afc3a92e336462 100644
--- a/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.idl
+++ b/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.idl
@@ -17,5 +17,5 @@ partial interface HTMLCanvasElement
     // (and is not undefined or null). The binding must ignore this.
     // Related spec issue: https://github.com/whatwg/html/issues/595
     [RaisesException] RenderingContext? getContext(DOMString contextId, [PermissiveDictionaryConversion] optional CanvasContextCreationAttributes attributes);
-    [RuntimeEnabled=ExperimentalCanvasFeatures, RaisesException] OffscreenCanvas transferControlToOffscreen();
+    [RuntimeEnabled=ExperimentalCanvasFeatures, RaisesException, MeasureAs=OffscreenCanvas] OffscreenCanvas transferControlToOffscreen();
 };