flacky test - LauncherAppBrowserTest.DragOffShelf

Still, this test is flacky.
Speed up animation makes this test more reliable.

R=skuhne@chromium.org
BUG=320143
TEST=browser_tests

Review URL: https://codereview.chromium.org/87303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237957 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237957