Revert "Convert tabcontentscontainer to use a vbox instead of a fixed. The"

This reverts commit r30843.

This broke the findbar/download shelf animation optimization.  I
originally was doing this as part of fixing issue 26495, but I
found a different way.

TBR=estade

Review URL: http://codereview.chromium.org/366009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31003 0039d316-1c4b-4281-b951-d872f2087c98