HTMLImageElement: do not use fallback content for ImageDocument
After https://codereview.chromium.org/1879793003,
ImageLoader::hasPendingActivity() became false for ImageDocument and thus
HTMLImageElement::selectSourceURL() didn't consider the image as still loading
during load (|imageStillLoading| became false).
This caused ImageDocument not to be displayed progressively.
This CL makes HTMLImageElement to use primary content for ImageDocument.
This causes ImageDocument not to be replaced with fallback content, but I
expect that is more acceptable than images not displayed progressively.
BUG=632495
Review-Url: https://codereview.chromium.org/2343613002
Cr-Commit-Position: refs/heads/master@{#420432}
5 files changed
