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This will measure the time from Ctrl-Shift-/ keystroke to dialog show.
This CL works with the non-mojo-app shortcut viewer we're using today.
When we switch to the app version we'll need to pass the keystroke
time over mojo to the app, perhaps via a Toggle() method. Adding the
mojo Toggle() interface is tracked in crbug.com/847615

Metric is Keyboard.ShortcutViewer.StartupTime

Bug: 847615, 847613
Test: added to ash_unittests, manually checked chrome://histograms
Change-Id: I425f44d0b71543f225d73857365d050be0e1b446
Reviewed-on: https://chromium-review.googlesource.com/1087779
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564952}