Remove document().page() null checks in MediaControls

These were needed in WebKit when the render theme was accessed via the
page, but that is no longer the case, these null checks are the only
bits in MediaControls that access document().page() at all.

It's possible that some code deeper down assumes that the page is not
null and will now begin to crash, but a quick scan of Source/core/
didn't reveal anything obvious.

BUG=341813

Review URL: https://codereview.chromium.org/190463002

git-svn-id: svn://svn.chromium.org/blink/trunk@168855 bbb929c8-8fbe-4397-9dbb-9b2b20218538