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The original tests pass non-thread-safe objects across threads which
could easily break.

This CL:
1. Adds 'storage_partition_test_utils.h/cc'.
2. Adds |IOThreadSharedURLLoaderFactoryOwner| which lives on UI thread
   and owns a |SharedURLLoaderFactory| on IO thread.
3. Moves related tests into 'storage_partition_impl_browsertest.cc' to
   make sure they run w/ and w/o Network Service.

This patch doesn't have behavior change.

Bug: 826869
Change-Id: I23d636b079d72627f8944dd61892d72f175e0c7d
Reviewed-on: https://chromium-review.googlesource.com/1033806
Commit-Queue: Chong Zhang <chongz@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556136}