Add user-activity-prediction-ml-service to chrome://flags

This flag allows users to manually enable the ML Service version of the
Smart Dim ("user activity prediction") display dimming model.

This is a temporary flag. This feature should roll out in M74.

Bug: 933207
Test: unit_tests --gtest_filter=AboutFlag*
Change-Id: I0473737c8081574b9db515ba0bdf71fc8797ecc9
Reviewed-on: https://chromium-review.googlesource.com/c/1475319
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633434}
5 files changed