"the content must also be accessible via toDataURL and toBlob on the <canvas> element."

I'm confused. If an HTMLCanvasElement has already been transferred to OffscreenCanvas, 
do we still allow toBlob/toDataURL happen? 

If so, then this part of exception thrown
(https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp?q=toBlob&sq=package:chromium&l=795)
is not right?

We're backtracking on this. esprehn@ made me realize that a lot of existing frameworks count on the fact that canvas contents are scrapeable from the DOM. Making this work is not hard and  it's inexpensive in terms of performance and memory.

Clarification: we're backtracking on the decision to throw an exception

 Issue 655177  has been merged into this issue.

junov@: So I mark this feature to be blocking the shipment of OffscreenCanvas.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/20ba52f0f6249c418d90244e3b7b1d8b317c30a4

commit 20ba52f0f6249c418d90244e3b7b1d8b317c30a4
Author: junov <junov@chromium.org>
Date: Mon Nov 14 23:13:40 2016

Synchronize OffscreenCanvas content with the placeholder canvas

This change solves the problem of propagating frames committed by
an OffscreenCanvas all the way to the placeholder canvas, whether it
be in the same script context or not.

This allows the placeholder canvas to be used as an image source,
and to be printed.

BUG= 655278 
TBR=kbr@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2493673002
Cr-Commit-Position: refs/heads/master@{#431953}

[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-placeholder-image-source-with-worker.html
[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-placeholder-image-source.html
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-transferControlToOffscreen.html
[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/LayoutTests/printing/offscreencanvas-2d-printing-expected.html
[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/LayoutTests/printing/offscreencanvas-2d-printing.html
[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/LayoutTests/printing/offscreencanvas-webgl-printing-expected.html
[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/LayoutTests/printing/offscreencanvas-webgl-printing.html
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/bindings/core/v8/ScriptValueSerializer.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/bindings/core/v8/serialization/V8ScriptValueDeserializer.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/bindings/core/v8/serialization/V8ScriptValueSerializer.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/bindings/core/v8/serialization/V8ScriptValueSerializerTest.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/core/html/HTMLCanvasElement.h
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.h
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModuleTest.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/BUILD.gn
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/graphics/Image.h
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcher.h
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.cpp
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/graphics/OffscreenCanvasFrameDispatcherImpl.h
[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/graphics/OffscreenCanvasPlaceholder.cpp
[add] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/graphics/OffscreenCanvasPlaceholder.h
[modify] https://crrev.com/20ba52f0f6249c418d90244e3b7b1d8b317c30a4/third_party/WebKit/Source/platform/graphics/StaticBitmapImage.h

Yay!