Expose the poster image URL to WebMediaPlayer subclasses

This is the Blink change for exposing the poster image to Chrome for Android.

BUG=333260

Review URL: https://codereview.chromium.org/131763011

git-svn-id: svn://svn.chromium.org/blink/trunk@167054 bbb929c8-8fbe-4397-9dbb-9b2b20218538