views: simplify ViewTracker

You were right! All consumers only want a single View, so simplify
ViewTracker to only handle a single view.

BUG=727934
TEST=covered by tests
R=msw@chromium.org

Change-Id: I9c504e616bec9f42f5de4b1b9c6e20c970e95002
Reviewed-on: https://chromium-review.googlesource.com/522388


Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#477036}