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The old keyboard overlay has been removed in M72 and
the flag kAshEnableKeyboardShortcutViewer is also removed.

Bug: 778874
Test: manual
Change-Id: I6ab9c350728c1e81f2d2437ac089187c4ab604c6
Reviewed-on: https://chromium-review.googlesource.com/c/1371073
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615567}