Recommend use of base::Features in about flags docs.

This corresponds to my discussion with Chrome Eng Review during
design / development of the base::Feature API that it should be the
recommended way going forward.

BUG=none

Review-Url: https://codereview.chromium.org/2400993003
Cr-Commit-Position: refs/heads/master@{#424448}
1 file changed