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Currently one can activate the tab switch button with shift-space etc.
This CL restricts it to only space, no modifiers.

Bug: 780835
Change-Id: I49b50d9d693e6201d7c3e74c91929bf71280865f
Reviewed-on: https://chromium-review.googlesource.com/1163991Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Kevin Bailey <krb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581122}