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ThreadableLoaderClient has not been GarbageCollected mainly for
WorkerThreadableLoader. Now WorkerThreadableLoader is gone, so we can
make it an on-heap class. Doing so brings several benefits:

 - This is a blocker of removing ThreadableLoader after OOR-CORS is
   enabled, as ResourceClient is a GarbageCollectedMixin.
 - As ThreadableLoader holds a reference to its client as a raw
   pointer, each user needs to guarantee to cancel the loader properly
   before the client gets invalid. As most of clietns are already
   on-heap, this requires pre-finalizer for each client. Also, failing
   to keep the contract leads to crashes which are very hard to
   investigate.

Bug: 900506
Change-Id: I67b3e3dab7e0600f06da5222d458ff2d81c36c7b
Reviewed-on: https://chromium-review.googlesource.com/c/1335056Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607970}