This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.
There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.
This allows HTMLCanvasElement and WebGL rendering contexts to share
their lifetime, as JS allows them to arbitrarily access them at any
time and WebGLRCB.canvas expects a non-null return value.
