Experimental app list slide animation is now horizontal, not vertical.
This makes the lack of proper clipping of WebContents more noticeable,
but it's behind a flag so fixing it isn't urgent.
Note that the animation always slides pages to/from the left side
(previously the top). In the future, the pages will be positioned in the
correct order.
BUG=376068
Review URL: https://codereview.chromium.org/
305433002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272945 0039d316-1c4b-4281-b951-d872f2087c98