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This will allow ContentHash caching to be (trivially)
implemented in the future in ContentVerifier::GetContentHash().
This CL also makes extension load at startup to queue all inflight
requests to load verified_contents and computed_hashes reading
from disk. This is because inflight requests will be served by
single file read.

Summary of changes:
The primary method is ContentVerifier::GetContentHash(). This
method is responsible for retrieving ContentHash on IO thread. It
uses ContentVerifier::HashHelper to manage thread hops and such.
HashHelper is not thread safe and it uses static method + weak
pointers to safely call ContentHash methods on blocking thread.

ContentHash class is responsible for fetching verified_contents.json
from network, using ContentHashFetcher. ContentHashFetcher
manages its own lifetime and is no longer ref-counted. Also,
ContentHashFetcherJob is no longer required.
ContentHashFetcher is currently an implementation detail of
ContentHash, and can be further restricted so that it cannot be
instantiated outside of ContentHash.
Because of async nature requirement of net fetch, ContentHash::Create
is now async.

Since ContentHash instance can travel between sequences, they are
RefCountedThreadSafe now.

ContentVerifier::VerifyFailed used to be (also) called when we fail to
retrieve hashes on disk and it would fall back to ContentHashFetcher
for fetching the hashes. As GetContentHash currently takes care of
this through ContentHash::Create, ContentVerifier is only made aware
of failures on when hash mismatch occurs, simplifying things further.

Bug: 796395
Change-Id: I7eb6bf93daba908258ae604a8cd45ea643b4e00d
Reviewed-on: https://chromium-review.googlesource.com/933863
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545623}