Fix case where install puts two copies of the extension toolbar on the shelf.
Fix case where install puts two copies of the extension toolbar on the shelf.

BUG=crbug.com/14941
TEST=install an extension, verify that only one toolbar is visible

Review URL: http://codereview.chromium.org/147026

git-svn-id: http://src.chromium.org/svn/trunk/src@19061 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 6709d4088820ab75352d9ad1784b6f5d2e006504
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