Change base::JSONReader::Read()/ReadAndReturnError() to return Optional.

Update unit tests and parts of base/ to reflect this change. Convert
existing callers that did not get updated to use the deprecated version
of the API.

BUG=925165
TBR=davidben@chromium.org,pfeldman@chromium.org,rdevlin.cronin@chromium.org,sdefresne@chromium.org,xiyuan@chromium.org,yucliu@chromium.org

Change-Id: If0cffdb24db7a7e829824d26cb300175cfcbfa23
Reviewed-on: https://chromium-review.googlesource.com/c/1435825
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632920}
38 files changed