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When the Tab Grid is visible, set |scrollEnabled| to YES to allow the
user to switch between recent tabs, incognito, and regular browser, and
set to NO otherwise (when the Thumb Strip or Tab Strip are visible).

Change-Id: I61bd079d50c9ab27b7970dcacaf149438595a269
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445881
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Roberto Moura <mouraroberto@google.com>
Cr-Commit-Position: refs/heads/master@{#813659}