Add feature flag for scrollbar scroll injecting gestures

Given that there were a couple regressions the first time I changed this
behavior for scrollbar presses (see crrev.com/1580588 and the revert,
crrev.com/1616341), I'm introducing a feature flag so that we can
have a kill switch.

R=bokan@chromium.org, dcheng@chromium.org, dtapuska@chromium.org

Bug: 954007
Change-Id: I635c5f39a798986c820287c47474f8b80a473307
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617140
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662075}
2 files changed