Bug 964559 - Remove MOZ_OGG. r=doublec,ted

We are unable to maintain all the --disable-foo build-time options
for the media code. Just support ogg unconditionally.
Bug 964559 - Run ogg mochitests unconditionally. r=doublec

Now that support is always enabled, we can remove the conditionals
on the ogg-dependent tests.

The canplay test previously run only if ogg was disabled is updated
to verify that the runtime pref works when set to a non-default
'false' value.
Bug 964559 - Expect empty string for canPlayType failure. r=doublec

As the mpeg and dash tests already knew, canPlayType() returns an
empty string ("") instead of "no" for unsupported formats. The
can_play_no_foo tests have been broken for some time.
Bug 976211 - pref off test_framebuffer.html. r=kinetik

This test no longer passes, the failure masked by bug 975640.

Since this feature is deprecated and disabled anyway, just
make the test do nothing if the controlling pref is false.
That way we can re-enable tests, but if the feature is
restored the bitrot in the implementation will be flagged.

Also change the channel count back to 6, since we're reporting
the native value again.