When setting a track .src to "", we would resolve the URL to a null URL,
which matched the default initialized HTMLTrackElement::url_, meaning
that readyState was not advanced and no 'error' event fired in this
case.

Extend the "url == url_" check with a check for readyState differing from
'none'. Also rewrite HTMLTrackElement::getReadyState to avoid using
EnsureTrack().

Bug: 811713
Change-Id: Icd190492639c53e09a7e018202b69ac6e919a94c
Reviewed-on: https://chromium-review.googlesource.com/916004
Commit-Queue: Fredrik Söderquist <[email protected]>
Reviewed-by: srirama chandra sekhar <[email protected]>
Cr-Commit-Position: refs/heads/master@{#536685}