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This CL adds ChromeOS.SystemTray.TimeToClick metrics to both old
SystemTray and UnifiedSystemTray.
TimeToClick is the amount of time a user took from clicking on the
button in status area, to clicking on an item in the system tray bubble.
The system tray view installs TimeToClickRecorder as PreTargetHandler,
and it calls SystemTray::RecordTimeToClick.

TEST=manual(chrome://histograms)
BUG=834462

Change-Id: Id7b599faf9bf53279be228816c39b05170db0d90
Reviewed-on: https://chromium-review.googlesource.com/1018120Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552930}