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The ScopedSubresourceFilterFeatureToggle remains, but is reduced to be
responsible for overriding only the base::FeatureList state.

The ScopedSubresourceFilterConfigurator is introduced as the new way for
overriding the configuration returned by GetActiveConfigurations(), and has the
following advantages:
  -- It does so regardless of the current field trial, feature list, and/or
     variation parameters states, thus the configuration can be changed
     multiple times during a single test.
  -- It can be used without a global base::FieldTrialList singleton, or setting
     up variations parameters.
  -- It will allow setting multiple simultaneous configurations conveniently
     without needing to first encode them as variation parameters.

BUG=708181

Review-Url: https://codereview.chromium.org/2838193002
Cr-Commit-Position: refs/heads/master@{#468939}