v8bindings: Add missing default values in *.idl files

{Canvas,Offscreen}RenderingContext APIs are missing default values
of IDL dictionary types.  Adds them.

No behavioral change.

Bug: 839389
Change-Id: I03ab9c8d7b568174a31561a54f698fad2a6812aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153169Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759949}

third_party/blink/renderer/modules/canvas/htmlcanvas/html_canvas_element_module.idl

@@ -22,7 +22,7 @@ typedef (CanvasRenderingContext2D or
     // requires throwing TypeError if the incoming argument is not an object type
     // (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 CanvasContextCreationAttributesModule attributes);
+    [RaisesException] RenderingContext? getContext(DOMString contextId, [PermissiveDictionaryConversion] optional CanvasContextCreationAttributesModule attributes = {});
 
     [CallWith=ExecutionContext, RuntimeEnabled=SurfaceEmbeddingFeatures,
      RaisesException, MeasureAs=OffscreenCanvas] OffscreenCanvas transferControlToOffscreen();

third_party/blink/renderer/modules/canvas/offscreencanvas/offscreen_canvas_module.idl

@@ -13,5 +13,5 @@ enum OffscreenRenderingContextType { "2d", "webgl", "webgl2", "webgl2-compute",
 [
     ImplementedAs=OffscreenCanvasModule
 ] partial interface OffscreenCanvas {
-    [CallWith=ExecutionContext, RaisesException] OffscreenRenderingContext? getContext(OffscreenRenderingContextType contextType, optional CanvasContextCreationAttributesModule attributes);
+    [CallWith=ExecutionContext, RaisesException] OffscreenRenderingContext? getContext(OffscreenRenderingContextType contextType, optional CanvasContextCreationAttributesModule attributes = {});
 };