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Revert of Change ScopedFeatureList to overrides FeatureList not reset (patchset #6 id:140001 of https://codereview.chromium.org/2834583002/ )

Reason for revert:
Mac ASAN bots reporting use-after-free errors.

Original issue's description:
> Change ScopedFeatureList to overrides FeatureList not reset
>
> 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}
> Committed: https://chromium.googlesource.com/chromium/src/+/9c04ed553bd7abe820a6a93c5e8981e6738881a9

TBR=isherman@chromium.org,asvitkine@chromium.org,thakis@chromium.org,chaopeng@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=713390

Review-Url: https://codereview.chromium.org/2850073002
Cr-Commit-Position: refs/heads/master@{#468263}