Fix custom cursors on Mac.  Null and 0 turn out not to be opposites.

I also added an assertion to webcursor_mac.mm that is probably a bit
redundant with the one in AppKit, but this way the assertion is more
visible to someone reading this file.

BUG=16346
TEST=load a page that uses a custom css cursor.
R=pinkerton

Review URL: http://codereview.chromium.org/155320

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@20327 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed