Don't attempt to set the cursor to a NULL cursor which in turn gets replaced with an ARROW cursor
when we receive a non client mouse message for a window which has no delegate like the RootWindow.

It seems like allowing the OS to do its thing here should be the right thing to do.

BUG=336297
R=sadrul@chromium.org, sky@chromium.org, sadrul, sky

Review URL: https://codereview.chromium.org/139343009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246201 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246201