[Cast, Android] Added isRemote() method to the WebMediaPlayer interface

Needed by https://codereview.chromium.org/644463003/

BUG=420665

Review URL: https://codereview.chromium.org/645503002

git-svn-id: svn://svn.chromium.org/blink/trunk@183467 bbb929c8-8fbe-4397-9dbb-9b2b20218538