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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}