Move WebLayer handling out of WebMediaPlayerClientImpl and into HTMLMediaElement.

Moving the WebLayer handling out of WebMediaPlayerClientImpl as a step
towards removing the MediaPlayer layer. This doesn't introduce any
change in functionality. It just moves code out of the MediaPlayer
layer.

BUG=310466
TESTS=All existing LayoutTests still pass.

Review URL: https://codereview.chromium.org/92613002

git-svn-id: svn://svn.chromium.org/blink/trunk@163119 bbb929c8-8fbe-4397-9dbb-9b2b20218538