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It used to be used for file URLs and TransportSecurityStatePersister as
well, but this CL moves those over to using the task scheduler. Can't
just always use that for the cache as well due to the cache needing a
SingleThreadTaskRunner, and getting one of those from the task scheduler
is a bit more resource intensive.

BUG=734199
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2977693003
Cr-Commit-Position: refs/heads/master@{#485919}