[ES6 modules] Make Modulator regular GCollected / not mixin

Before this CL, Modulator was a GCMixin, but there was no reason that it needed to be a mixin.
This CL changes it to a regular GarbageCollectedFinalized obj.

This CL prepares Modulator/ModuleMap/ModuleMap::Entry/ModuleScript to be converted into
TraceWrapperBase.

BUG=594639

Review-Url: https://codereview.chromium.org/2788533002
Cr-Commit-Position: refs/heads/master@{#460696}
4 files changed