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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