Drop [LegacyImplementedInBaseClass] from WindowMediaSource IDL interface

Drop [LegacyImplementedInBaseClass] from WindowMediaSource IDL interface.
It is not needed, it builds fine without it.

R=haraken
BUG=344224

Review URL: https://codereview.chromium.org/163143003

git-svn-id: svn://svn.chromium.org/blink/trunk@167248 bbb929c8-8fbe-4397-9dbb-9b2b20218538