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This CL modifies the feature flag overscroll navigation so that users
can enable/disable navigation sheet as an option.

Bug: 937946
Change-Id: Id7bf39054d561927be74262331226ce112ef7caf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1694184
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676326}