Replace HTMLMediaElement::player() calls with webMediaPlayer()

Removed player() calls and use webMediaPlayer() instead since it is enough to check the MediaPlayer existance.

Review URL: https://codereview.chromium.org/568763002

git-svn-id: svn://svn.chromium.org/blink/trunk@182042 bbb929c8-8fbe-4397-9dbb-9b2b20218538