[Blink>Paint] Document element cannot always omit isolation

The previous assumption that nothing can be the backdrop of the document
element's stacking context was incorrect, because document background doesn't
paint as a part of the stacking context but as canvas backdrop instead.

Note that only non-composited blending elements are affected by this bug.
This CL changes behavior to match composited code path, which also matches
Firefox behavior.


Review-Url: https://codereview.chromium.org/2564423003
Cr-Commit-Position: refs/heads/master@{#438298}
5 files changed