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Instead of creating CHR and then calling CHR::Init(), create it
in one go (in a blocking allowed thread). The instance is immuatable and
stored inside ContentVerifyJob, hence CHR doens't need to be
RefCountedThreadSafe anymore.

ContentHashReader only needs extension related information on creation,
so move all such params from the class members (to ContentVerifyJob).

Bug: 796395
Change-Id: Ia65938d96e99d5c95562c75920cd614935026240
Reviewed-on: https://chromium-review.googlesource.com/917007Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538994}