Bug 903212 - Remove activity tracking from nsJSContext. r=smaug

This only has an impact when we enable explict compartmental GCs, which have
been preffed off for a while.
Bug 903212 - Add a GC when popping a script entry point. r=mrbkap

I think that this is more or less isomorphic with the MaybeGC we do in
ScriptEvaluated in the cases that matter. And doing it in a spec-defined
place is helpful in getting rid of spec-nonsensical machinery.
Bug 903212 - Remove ScriptEvaluated, and simplify callers. r=mrbkap
Bug 903212 - Remove nsIXPCScriptNotify machinery. r=mrbkap
Bug 903212 - Remove explicit compartment GC machinery. r=smaug
Bug 905062: Clean up fake source for getUserMedia r=padenot