HTMLMediaElement should not override childShouldCreateRenderer

HTMLMediaElement was using childShouldCreateRenderer to prevent children
from creating renderers unless it had controls enabled. Instead we should
just ensure there's always a ShadowRoot and then children will not get
rendered since they have no insertion point. This makes HTMLMediaElement
behave in a way that models what a custom element implementation of it
would have done too.

This removes the last user of childShouldCreateRenderer that is not SVG.

Review URL: https://codereview.chromium.org/125633002

git-svn-id: svn://svn.chromium.org/blink/trunk@164666 bbb929c8-8fbe-4397-9dbb-9b2b20218538