Make HttpCache backend creation not take a thread.

Just rely on the backend default instead.
This also strips all the wiring of it through higher-level methods.

Bug: 747020
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Change-Id: I16082ebff0a0a786dbcb5964b228a3772938f1dc
Reviewed-on: https://chromium-review.googlesource.com/585152


Reviewed-by: Selim Gurun <sgurun@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489654}