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This CL adds the following:
  - Two new APIs on FeatureList to be used during initialization. One to
associate a field trial for reporting purposes when the feature is forced
from the command line and the other to override the feature state via a
field trial.
  - Passing the FeatureList instance to VariationsService during browser
start up.
  - Extension of VariationsService (and associated proto changes) to
invoke the two above APIs, when processing variations with the new
proto fields.
  - A new API on FieldTrial to get the group name of a field trial without
activating it, used by VariationsService when association a field trial.

BUG=526169

Review URL: https://codereview.chromium.org/1306653004

Cr-Commit-Position: refs/heads/master@{#351199}