dnt: Add flag to mirror gestures when in RTL

When this flag is enabled and if Chrome's UI is in an RTL layout (i.e.
using a right-to-left language like Arabic), navigation gestures will
flip the meaning of the edges, i.e. treat the right edge as "back" and
the left edge as "forward". (Note: only 3-button mode currently
navigates forward from the forward edge. Gestural navigation navigates
back from both edges but doesn't show a transition animation from the
"forward" edge).

Note: this CL implements the flag and passes the right value to the
handler but animations and UI are still broken for the right edge.
Followup CLs will fix these issues.

Bug: 331778964
Change-Id: I7ca92da85a58f66f8cfea0adccafdca5dfe7bdd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5692438
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Lijin Shen <lazzzis@google.com>
Reviewed-by: William Liu <liuwilliam@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1328281}