Add use counters for recently launched canvas features

third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // https://html.spec.whatwg.org/multipage/scripting.html#the-offscreencanvas-interface
 
 [
     Constructor([EnforceRange] unsigned long width, [EnforceRange] unsigned long height),
     Exposed=(Window,Worker),
     RuntimeEnabled=ExperimentalCanvasFeatures,
     MeasureAs=OffscreenCanvas,
 ] interface OffscreenCanvas : EventTarget {
     [EnforceRange] attribute unsigned long width;
     [EnforceRange] attribute unsigned long height;
 
     [CallWith=ScriptState, RaisesException] ImageBitmap transferToImageBitmap();
-    [RaisesException, CallWith=ScriptState] Promise<Blob> convertToBlob(optional ImageEncodeOptions options);
+    [MeasureAs=OffscreenCanvasConvertToBlob, RaisesException, CallWith=ScriptState] Promise<Blob> convertToBlob(optional ImageEncodeOptions options);
 };