GitLab

Resorting the code into snapshot method as to make easier to understand.
Now snapshot will effectively ask for a GetImage in case that the context
is ImageBitmapRenderer.

Adding some comments to html_canvas_element.h to explain the types of
contexts that can be into the html_canvas_element and adding future todo.

Adding test to validate that getDataURL works on the bitmaprenderer context.

Bug: 932266, 838108
Change-Id: I96aa25a3db624a2ddf457ae7349e345955e9ad50
Reviewed-on: https://chromium-review.googlesource.com/c/1481107Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Juanmi Huertas <juanmihd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635634}