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. Content moves loading from PreCreateThreads() to
  PreEarlyInitialization() (or right after calling
  BrowserMainParts::PreEarlyInitialization()
  . This necessitated a change for cast to have content not init
    FieldTrials.
. Moves creation of FieldTrial in tests from
  TestBlinkWebUnitTestSupport to UnittestTestSuite. This is so code
  run after that (but before TestBlinkWebUnitTestSupport) has the
  FeatureList setup.

BUG=800357
TEST=none

Change-Id: I50e121f4c5654288571715b0e72bf04db340daf1
Reviewed-on: https://chromium-review.googlesource.com/925685Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537817}