HTMLMediaElement::webMediaPlayer() should never be null if m_readyState >= HAVE_METADATA (media engine error case)

BUG=382721

Review URL: https://codereview.chromium.org/330593005

git-svn-id: svn://svn.chromium.org/blink/trunk@176993 bbb929c8-8fbe-4397-9dbb-9b2b20218538