Remove WebMediaPlayerClient{Impl}::needsWebLayerForVideo() as compositing is always enabled.

BUG=335345
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/206283004

git-svn-id: svn://svn.chromium.org/blink/trunk@169895 bbb929c8-8fbe-4397-9dbb-9b2b20218538