MediaStream API: MediaStreamTrack can now belong to more than one Stream.
MediaStream::addTrack now adds the track directly instead of cloning it
first.

This patch has already been committed as r162758, rolled back due to a
failing browsertest in r162808. The failing browsertest has been slightly
changed to cope with both the old and new behaviour, and this patch have
had one line changed to correct another found issue.

BUG=294145

Review URL: https://codereview.chromium.org/95183003

git-svn-id: svn://svn.chromium.org/blink/trunk@162886 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Switch HTMLCanvasElement over to new-style ExceptionState.

Set up the calling context on toDataURL()'s incoming ExceptionState
instead, for tidier generation of a possible SecurityError exception.

To support throwing context-sensitive SecurityErrors, extend
throwSecurityError() to also embellish its incoming messages.

R=
BUG=270033

Review URL: https://codereview.chromium.org/96203003

git-svn-id: svn://svn.chromium.org/blink/trunk@162887 bbb929c8-8fbe-4397-9dbb-9b2b20218538