GitLab

Introduce the user action
WindowSelector_ActiveWindowChanged to track
the number of overview mode sessions in which
the user selected a window that is different
from the previously-active window.

BUG=486900
TEST=WindowSelectorTest.ActiveWindowChangedUserAction*

Review URL: https://codereview.chromium.org/1138253002

Cr-Commit-Position: refs/heads/master@{#329915}