Move ThreadableLoader to Oilpan heap (2/3)

This change makes ThreadableLoader GarbageCollectedFinalized. Apart from
trivial changes, this CL merges WorkerThreadableLoader,
ThreadableLoaderClientWrapper and WorkerThreadableLoader::Bridge.

BUG=587663

Review-Url: https://codereview.chromium.org/2181243002
Cr-Commit-Position: refs/heads/master@{#409736}
26 files changed