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The current situation is that using ScopedFeatureList resets to an
empty feature list and then enables/disables an explicit list of
features.

That's never what you want for browser tests (or other higher-level
tests) since it effectively overrides higher-level test configurations
(e.g. those in fieldtrial_testing_config.json, or a bot set up to
specifically test a feature).

In this patch:

1. Keep SFL::Init, SFL::InitWithFeatureList,
   SFL::InitFromCommandLine reset to empty list but add warning to
   remind developer should use them with care.
2. Change SFL::InitAndEnableFeature, SFL::InitAndDisableFeature and
   SFL::InitWithFeatures to not reset but override current FeatureList
   with given enables/disables.

We also add unit tests for ScopedFeatureList.

BUG=713390

Review-Url: https://codereview.chromium.org/2834583002
Cr-Commit-Position: refs/heads/master@{#468210}