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Each Web Worker runs on a separate worker thread, and each worker has an
associated URLLoaderThrottleProvider.  When spawning a nested worker,
the outer worker's throttle provider is cloned on the outer worker's
thread and then used on the inner worker's thread.

To accomodate this scenario:

 - Postpone the creation of DataReductionProxyThrottleManager until a
   moment when we are running on the target thread.  This makes sure
   the DataReductionProxyThrottleConfigObserver binding is used on the
   same thread that it's created on.

 - Postpone the binding of DataReductionProxy until a moment when we
   are running on the target thread.  DataReductionProxyPtrInfo
   serves as a holder of the interface request that can move between
   threads.

Bug: 942011
Change-Id: Ieed17fb499bbcb0545e3e0323f751c1ea55a33d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565873Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Wojciech Dzierżanowski <wdzierzanowski@opera.com>
Cr-Commit-Position: refs/heads/master@{#654846}