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To use the FeatureEngagementTracker to observe the New Tab and related
actions, the NewTabFeatureEngagementTracker class was created.

Things that were changed in this CL:
* Added the NewTabInProductHelp trial to the fieldtrial json
* Added a pref to track the user's total active session time
* Created a NewTabFeatureEngagementTracker class, which contains
methods to notify the IPH server when events happen and if the help
UI should be triggered.

Related CLs:
* The Observer for DesktopSessionDurationTracker
  http://crrev.com/d69292f
* Event constants that are used in NewTabFeatureEngagementTracker
  http://crrev.com/bdf5c97
* Using the NewTabFeatureEngagementTracker
  https://chromium-review.googlesource.com/c/549074/
* Addition to fieldtrial_testing.json
  https://chromium-review.googlesource.com/c/566335/

Bug: 734132
Change-Id: I8654ef001f3fb50fbd8bb8c4f215dffcfe403762
Reviewed-on: https://chromium-review.googlesource.com/549057
Commit-Queue: Catherine Chung <catherinechung@google.com>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Pam Greene <pam@chromium.org>
Reviewed-by: Anthony Vallee-Dubois <anthonyvd@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488293}