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This is part 3 of 6 for FieldTrial refactoring for WebView

Part 1: https://chromium-review.googlesource.com/c/562098/
Part 2: https://chromium-review.googlesource.com/c/561920/
Part 3: https://chromium-review.googlesource.com/c/561922/
Part 4: https://chromium-review.googlesource.com/c/561980/
Part 5: https://chromium-review.googlesource.com/c/562417/
Part 6: https://chromium-review.googlesource.com/c/562021/

This CL creates the VariationsFieldTrialCreator class. This class
contains the code previously in VariationsService that
is needed by both Chrome and WebView, mainly the portions
needed to call CreateTrialsFromSeed.  The VariationsService
class now contains an instance of this new class.

BUG=678288

Change-Id: Ic4340d5a0396cca56892a3b541e4cf6115bb8afb
Reviewed-on: https://chromium-review.googlesource.com/561922
Commit-Queue: Kyle Milka <kmilka@google.com>
Reviewed-by: Alexei Svitkine (slow) <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486590}