[CSS Blending] When applying mix-blend-mode on an element,
its contents and its children should blend as a whole with the
backdrop.

BUG=229615

Review URL: https://codereview.chromium.org/104663003

git-svn-id: svn://svn.chromium.org/blink/trunk@164026 bbb929c8-8fbe-4397-9dbb-9b2b20218538