Improve media-related exception messages

The two in HTMLMediaElement were just wrong.

Add a video-currentTime-set-with-controller.html test as that code path
was apparently untested before.

BUG=270033

Review URL: https://codereview.chromium.org/197213012

git-svn-id: svn://svn.chromium.org/blink/trunk@169314 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Expose CrossThreadPersistent assignment operators.

Allow the base class Persistent assignment operators to be used over
CrossThreadPersistent also.

R=haraken@chromium.org
BUG=347902

Review URL: https://codereview.chromium.org/196383024

git-svn-id: svn://svn.chromium.org/blink/trunk@169315 bbb929c8-8fbe-4397-9dbb-9b2b20218538