Check for valid webMediaPlayer() before using it.

      This fixes the underlying issue in bug 456312 and reverts the revert in
https://codereview.chromium.org/905023002, adding the fix and the layout tests back.

Manually tested by visiting youtube.com and clicking on videos, before the current video is finished. This requires the Audio EQ (HTML5 Audio Equalizer for Chrome) extension to be added and enabled.

BUG=456312, 313939

Review URL: https://codereview.chromium.org/905393002

git-svn-id: svn://svn.chromium.org/blink/trunk@190367 bbb929c8-8fbe-4397-9dbb-9b2b20218538