Revert of Directly composited images are images: ASSERT that (patchset #2 of https://codereview.chromium.org/491773002/)

Reason for revert:
Breaking android bots most of the time

Original issue's description:
> Directly composited images are images: ASSERT that
> 
> isDirectlyCompositedImage only applies to images. ASSERT that and
> make callers do all the isImage() testing.
> 
> BUG=405803
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180721

TBR=vollick@chromium.org,benm@chromium.org,noel@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=405803

Review URL: https://codereview.chromium.org/497143002

git-svn-id: svn://svn.chromium.org/blink/trunk@180779 bbb929c8-8fbe-4397-9dbb-9b2b20218538