Bug 1078354 - Part 1: Make sure we are not waking up an OfflineGraphDriver. r=jesup, a=bajaj

I still want to assert in the method instead of making it a no-op because I'm
going to work on AudioContext sleep and wakeup soon, and this will catch errors.
Bug 1078354 - Part 2: Don't try to measure a PeriodicWave size when an OscillatorNode is using a basic waveform. r=erahm, a=bajaj