Disable ClosingLastTabFinishesUnload and WindowCloseFinishesUnload which fail on Mac 64

BUG=301173
TEST=Mac 64 tester bot greens

Review URL: https://codereview.chromium.org/25190002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225908 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225908