flags: assign double-tap-to-zoom-in-tablet-mode

Bug: None
Change-Id: I54182aba33401292d5df0c382a6715a9cc76dc36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577936
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653244}