Make OffscreenCanvas 2D context support globalAlpha

third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.idl

 // Copyright 2016 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-offscreen-2d-rendering-context
 
 [
     Exposed=(Window,Worker),
     RuntimeEnabled=ExperimentalCanvasFeatures,
 ] interface OffscreenCanvasRenderingContext2D {
@@ skipping 45 matching lines @@
 
     // transformations (default transform is the identity matrix)
     void scale(unrestricted double x, unrestricted double y);
     void rotate(unrestricted double angle);
     void translate(unrestricted double x, unrestricted double y);
     void transform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
     void setTransform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
     void resetTransform();
 
     // compositing
+    attribute unrestricted double globalAlpha; // (default 1.0)
     attribute DOMString globalCompositeOperation; // (default source-over)
 
     // image smoothing
     attribute boolean imageSmoothingEnabled; // (default True)
     attribute ImageSmoothingQuality imageSmoothingQuality; // (default "low")
 
     // shadows
     attribute unrestricted double shadowOffsetX;
     attribute unrestricted double shadowOffsetY;
     attribute unrestricted double shadowBlur;
     attribute DOMString shadowColor;
 
     // pixel manipulation
     [RaisesException] ImageData createImageData(ImageData imagedata);
     [RaisesException] ImageData createImageData(double sw, double sh);
     [RaisesException] ImageData getImageData(double sx, double sy, double sw, double sh);
     [RaisesException] void putImageData(ImageData imagedata, double dx, double dy);
     [RaisesException] void putImageData(ImageData imagedata, double dx, double dy, double dirtyX, double dirtyY, double dirtyWidth, double dirtyHeight);
 
     [Exposed=Window] attribute DOMString filter;
 };
 
 OffscreenCanvasRenderingContext2D implements CanvasPathMethods;