Safeguard for webMediaPlayer() access in createImageBitmap() incase webMediaPlayer() and m_readystate of media element are out of sync

BUG=350571

Review URL: https://codereview.chromium.org/320843002

git-svn-id: svn://svn.chromium.org/blink/trunk@175943 bbb929c8-8fbe-4397-9dbb-9b2b20218538