Support drag&drop for startup pages

BUG=69686
TEST=Drag & drop individual or multiple startup pages, confirm D&D works


Review URL: http://codereview.chromium.org/7044136

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88936 0039d316-1c4b-4281-b951-d872f2087c98