Use MeasureAs and ImplementedAs in media IDL

This leaves the C++ code a little bit cleaner.

Also indent HTMLMediaElement.idl for consistency.

BUG=309364

Review URL: https://codereview.chromium.org/32103002

git-svn-id: svn://svn.chromium.org/blink/trunk@160119 bbb929c8-8fbe-4397-9dbb-9b2b20218538