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Before this change, URLLoaderInterceptor required using a nested RunLoop,
which isn't allowed on Android instrumentation tests (
DCHECK(IsTestImplementation()) fails in message_pump_android.cc).

Now URLLoaderInterceptor can optionally avoid the RunLoop by using asynchronous
initialization. To trigger this new behavior, the caller must pass in a
callback to be invoked after the interceptor is installed.

Change-Id: Ib76c66d9639d8a48dcf5d75fe1f9a6042e9eeda1
Reviewed-on: https://chromium-review.googlesource.com/c/1385121Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Dan H <harringtond@google.com>
Cr-Commit-Position: refs/heads/master@{#619997}