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This allows DeferredTaskHandler::RemoveMarkedSummingJunction to
assume that the graph lock is held, rather than having to acquire
it (recursively, if it's already held).

Since Oilpan finalizers cannot safely touch other heap objects (as
those object may be swept in the same GC cycle), the AudioNode
and AudioParam objects now hold a direct reference to the
DeferredTaskHandler associated with the audio context, which is
needed to acquire the appropriate lock.

Bug: 856641
Change-Id: Ied445a684d60b8beccb080b98867fcf80f967d21
Reviewed-on: https://chromium-review.googlesource.com/1182296
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585069}