[TTS] Default-enable AMP clicks promote to new tab

A previous CL added special treatment for AMP pages when they
are clicked on in the Contextual Search Overlay.  However the
functionality was behind a flag, which we never enabled.

This changes the AMP case to be behind a default-enabled flag that
will allow us to disable if needed.

BUG=645325

Review-Url: https://codereview.chromium.org/2714623002
Cr-Commit-Position: refs/heads/master@{#453291}