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This CL uses the NewTabFeatureEngagementTracker, which you can view
in this CL: http://crrev.com/7d54c1b52c11ef28e9a19f3c6a5a7a41a25fbf5d

The New Tab FeatureEngagementTracker will be notified if:
* The user presses the New Tab button
* The user presses Control+T
* The user uses the tabstrip context menu, window frame context menu,
  or the app menu to open a New tab.
* The user uses the omnibox to start a session in the same tab.
* The user focuses on the omnibox.

Bug: 734132
Change-Id: I5c9f3c394a2f2001b812a18e788fdc68220dcd20
Reviewed-on: https://chromium-review.googlesource.com/549074Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Catherine Chung <catherinechung@google.com>
Cr-Commit-Position: refs/heads/master@{#488771}