Move ImageResourceContent::BrokenCanvas to a better place

This function has no association with ImageResourceContent, so move it
to html_canvas_element.cc where it's only user resides.

Change-Id: I22e50d0f526f150957aec262452c7f1dfcf612e5
Reviewed-on: https://chromium-review.googlesource.com/1120332
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#572751}
3 files changed