Fix download shelf item overflow on os x.
Fix download shelf item overflow on os x.

This was regressed by the subview layout refactoring.

BUG=14672
TEST=download several items, resize window, make sure that no download item ever overlaps the "Show all downloads" link on the right.

Review URL: http://codereview.chromium.org/173048

git-svn-id: http://src.chromium.org/svn/trunk/src@24683 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 9e2421c154503919c56e3b71eb7ef107f3092d45
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser