[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.
BUG=582539,588809
Review URL: https://codereview.chromium.org/1727823002
Cr-Commit-Position: refs/heads/master@{#377803}
5 files changed
