Remove knowledge of layer opacity from HTMLMediaElement.

WebMediaPlayer implementations are capable of setting layer opacity themselves.

BUG=110814
R=acolwell@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/237743002

git-svn-id: svn://svn.chromium.org/blink/trunk@171720 bbb929c8-8fbe-4397-9dbb-9b2b20218538