MediaStream API: Cleaning up exceptions
Getting rid of some throwUninformativeAndGenericDOMException calls.
Also making sure RTCSessionDescription follows the standard.

Review URL: https://codereview.chromium.org/27235005

git-svn-id: svn://svn.chromium.org/blink/trunk@160189 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add UseCounter for HTMLSourceElement.media

https://www.w3.org/Bugs/Public/show_bug.cgi?id=19619

By counting the number of candidate <source> elements and the number
of non-matching media attributes, we can determine what the impact of
removing the attribute from spec and implementation would be.

BUG=310006

Review URL: https://codereview.chromium.org/32583007

git-svn-id: svn://svn.chromium.org/blink/trunk@160190 bbb929c8-8fbe-4397-9dbb-9b2b20218538