stop() can be called more than once in WebAudio

The W3C audio group has decided to allow stop() to be called more than
once.  The last call to stop() is the one that takes effect, unless
a previous call has caused the source to stop. No exception is thrown
in any case.

Removed test cases that expected multiple calls to stop to throw
exceptions.

BUG=324652

Review URL: https://codereview.chromium.org/140683005

git-svn-id: svn://svn.chromium.org/blink/trunk@165570 bbb929c8-8fbe-4397-9dbb-9b2b20218538