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It didn't make sense to use 'runtime_enabled_function_name'
to group runtime enabled features.
Plus, 'runtime_feature_name' was also used in few places. It
was so confusing.

So this CL unify them to use feature names to group contexts
and remove runtime_enabled_function_name() function.
This CL does not change generated code.

BUG=None

Review-Url: https://codereview.chromium.org/2578943002
Cr-Commit-Position: refs/heads/master@{#439088}