Add user actions for stacked tab mode

Add user actions for stacked tab mode

Add two user actions for stacked tab mode:

* StackedTab_EnteredStackedLayout is recorded when
  the tabstrip layout becomes stacked. This happens
  when the user is interacting with the tabstrip
  using touch and there is insufficient space to be
  able to display all tabs at a minimum
  touch-friendly width.

* StackedTab_DragActiveTab is recorded when the
  user drags the active tab (to scroll through
  the list of open tabs) while in stacked tab mode.

BUG=466103,481134
TEST=none

Review URL: https://codereview.chromium.org/1051923005

Cr-Commit-Position: refs/heads/master@{#327728}