Test setting string properties to null on <audio>, <video>, <source> and <track>

http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html

The spec doesn't have [TreatNullAs=NullString] for any of these
properties, so the added tests are actually wrong for mediaGroup,
poster and src. The implementation will be fixed in future commits.

BUG=none

Review URL: https://codereview.chromium.org/30693003

git-svn-id: svn://svn.chromium.org/blink/trunk@160195 bbb929c8-8fbe-4397-9dbb-9b2b20218538