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This takes a hint from r588692 and moves ownership of the
ProxyingURLLoaderFactory objects a user data object that hangs off of
the content::ResourceContext. This prevents these objects from outliving
this object on the IO thread. The previous ownership model was open to a
race condition between the destruction of the BrowserContext keyed
service ProxyingURLLoaderFactoryManager on the UI thread and the tasks
posted to the IO thread to destroy each ProxyingURLLoaderFactory. These
would be scheduled after the task to destroy the ResourceContext.

Change-Id: I600b1604526b644c3ac9e010d1cb82f4b3740925
Reviewed-on: https://chromium-review.googlesource.com/1211913Reviewed-by: Clark DuVall <cduvall@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589522}