Make |extends| option in custom element behave correctly

When null is given to |extends| while instantiating the
custom element, it should be the same case when |extends|
is not specified at all.

BUG=323882
TEST=fast/dom/custom/type-extension-is-null.html

Review URL: https://codereview.chromium.org/476713002

git-svn-id: svn://svn.chromium.org/blink/trunk@180811 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Directly composited images are images: ASSERT that

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

BUG=405803,406418

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180721

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

git-svn-id: svn://svn.chromium.org/blink/trunk@180812 bbb929c8-8fbe-4397-9dbb-9b2b20218538