Disable the user selection on the keyboard overlay.

BUG=none
TEST=Manually on chromebook


Review URL: http://codereview.chromium.org/7569006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@95626 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/keyboard_overlay.css

 body {
+  -webkit-user-select: none;
   background: #fff;
   margin: 0;
   padding: 0;