Directly composited images are images: ASSERT that

isDirectlyCompositedImage only applies to images. ASSERT that and
make callers do all the isImage() testing.

BUG=405803

Review URL: https://codereview.chromium.org/491773002

git-svn-id: svn://svn.chromium.org/blink/trunk@180721 bbb929c8-8fbe-4397-9dbb-9b2b20218538