Fix DnD jank
Fix DnD jank

The mouseover code I added to do *smart* tooltips was making DnD almost
unusable.

BUG=None

TEST=Drag thumbnails on the NNTP and they should move smoothly.

Review URL: http://codereview.chromium.org/149427

git-svn-id: http://src.chromium.org/svn/trunk/src@20413 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 82350a6ef8c65284e7309d1ac58a81619324e730
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