GitLab

Part of refactor to allow for a DetachedPanelStrip.
Refactor of Panel to know it's current panel strip will be a separate patch.

BUG=None
TEST=No new tests

Review URL: https://chromiumcodereview.appspot.com/9353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@120809 0039d316-1c4b-4281-b951-d872f2087c98