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This cl moves sync consent tests to the normal chromium build target.

Made fixes to the sync consent tests:
 * Added FakeGaia mixin to allow the call to ShowSigninScreenForTest to
   complete.
 * Updated the OOBE JS API call for updating language locale.
 * SyncConsentPolicyDisableTest now waits for user session to start
   because OOBE flow can finish before the Waiter started, causing the
   Waiter to wait forever.

Moved the is_official_build_ flag in WizardController to static scope
because a new WizardController instance is created as part of the login
flow. This meant that setting is_official_build_ to true during the
test would be overriden when the new WizardController instace is
created.

Added a helper function ForceOfficialBulidForTesting to WizardController
that temporarily overrides is_official_build_ during the execution
of a test.

Removes some WizardController friend classes that only needed access
to the private is_official_build_ flag.

TBR=antrim@chromium.org

Bug: 952388

Change-Id: I5bd87ef4a21b4a265bdde94656fc2380c770473a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570215
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651776}