Move deferred loading logic from WebMediaPlayerClientImpl to HTMLMediaElement.

BUG=350571
TEST=All layout tests still pass.

Review URL: https://codereview.chromium.org/192013002

git-svn-id: svn://svn.chromium.org/blink/trunk@169465 bbb929c8-8fbe-4397-9dbb-9b2b20218538