Update for new GMP API changes in FF34


Review - https://rbcommons.com/s/OpenH264/r/699/

The GMP API has changed for Firefox34.  Here are the pushes today:

https://hg.mozilla.org/integration/mozilla-inbound/rev/fd72e28c6d38
https://hg.mozilla.org/integration/mozilla-inbound/rev/1b3e953be799
https://hg.mozilla.org/integration/mozilla-inbound/rev/6712067462a9


For us this mostly just means that we need to use the callback->Error() instead of returning an error value on our API methods.  Only the gmp-openh264.cpp file is affected.