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Create a flag to implement keyboard shortcut viewer. This flag will be
removed once the feature is launched.

Bug: 778874, 768932
Test: test new flag with command line, and chrome://flags.
Change-Id: I7ea41f990059a52c4a3f008fff9f8b9ba30da938
Reviewed-on: https://chromium-review.googlesource.com/740943
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513097}