Move kDirectCompositionUnderlays feature from GL switch to Finch experiment.

We are ready to enable this feature. So let it run through Finch experiment
first. Without this CL change, it can still be enabled through the server side
config, but we want to make it clear that this feature is currently run under
Finch experiment, not just a regular GL switch.

Bug:868561

Change-Id: I438daa5e75d28e7e55e58f37723c648f3a4903a0
Reviewed-on: https://chromium-review.googlesource.com/c/1451816
Commit-Queue: Maggie Chen <magchen@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628903}
7 files changed