Fix hiding of the cursor in Presentation Mode in WebKit browsers

This PR is the fourth in a series of patches that will improve the presentation mode implementation. (The previous ones were #3752, #3763 and #3774.)

This patch makes sure that the cursor is hidden in Presentation Mode in WebKit browsers. For a description of the issue, see: http://code.google.com/p/chromium/issues/detail?id=103041.
*Note that this has been fixed upstream, but the fix hasn't yet made it into the current release version of Google Chrome.*