[Herb] Add metrics

* Adds metrics for the system back button, the return button, and the
  Open in Chrome button.  The SystemBack action records whether back
  was hit, while a histogram tracks specifically what happened when
  this happened.

* Adds a histogram that tracks why ChromeTabbedActivity was stopped.

* Edits the back pressed logic in ChromeTabbedActivity#handleBackPressed()
  so that it's possible to return false if the action really, really
  isn't handled.

* Recombines the two handleBackPressed methods.  They were separated
  initially because the system back behavior required it for one of
  the prototypes but that isn't the case anymore.


Review URL: https://codereview.chromium.org/1727823002

Cr-Commit-Position: refs/heads/master@{#377803}
5 files changed