Through code search on file ImagePattern.cpp, suspected CL in one of these:

https://chromium.googlesource.com/chromium/src/+/65bcd11753b2561dd86c6c95a778bc397ab54367
https://chromium.googlesource.com/chromium/src/+/b0f2b78559b1d731012d9808376c08b6ec256396

xlai@, could you please take a look and please help us to find correct owner if it is not related your changes.

I guess I know why the regression happens in Chromium revision 416628:417590 and yet my two CLs are outside that range. It's probably this CL
(https://chromium.googlesource.com/chromium/src/+/980562f70d53ff313d4f5cd885c70248d3dd902a) that's modifying the function when OffscreenCanvas is a
CanvasSourceImage; it may introduce a possibility that the image is null and the
status is normal. +cc junov.
Nevertheless, I got a CL that's fixing it now (https://codereview.chromium.org/2344573002/).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ce494afba26d386e562b8e8d84ef707d5c54bfc3

commit ce494afba26d386e562b8e8d84ef707d5c54bfc3
Author: xlai <xlai@chromium.org>
Date: Thu Sep 15 19:05:12 2016

Correct SourceImageStatus in OffscreenCanvas::getSourceImageForCanvas

The crash in constructor of ImagePattern may be due to the fact that
image passed in as its constructor argument is nullptr. By tracking
all the call trace backwards, we find the image creation originates
from BaseRenderingContext2D::createPattern, where the SourceImageStatus
indicates what corresponding action to do. But this SourceImageStatus
was wrongly set for OffscreenCanvas, in particular, in the function
OffscreenCanvas::getSourceImageForCanvas. This CL corrects it and makes
sure that if the image is null, BaseRenderingContext2D::createPattern
will return Image::nullImage() instead of nullptr.

BUG= 646654 

Review-Url: https://codereview.chromium.org/2344573002
Cr-Commit-Position: refs/heads/master@{#418927}

[add] https://crrev.com/ce494afba26d386e562b8e8d84ef707d5c54bfc3/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-empty-image-source.html
[modify] https://crrev.com/ce494afba26d386e562b8e8d84ef707d5c54bfc3/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp

This bug is already fixed.

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