[Chromoting] Fix keystone install script to use .pkg instead of .mpkg.

Iceberg builds .mpkgs, but Packages builds flat .pkgs. We switch to using
Packages because it can create "flag packages" that we can sign and 
signing is required for Mac OS X 10.8.

When we switched from .mpkg to .pkg, the keystone installer script was
not updated. So keystone installs are not happening.
Review URL: https://chromiumcodereview.appspot.com/10831041

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148655 0039d316-1c4b-4281-b951-d872f2087c98

remoting/host/installer/mac/Scripts/keystone_install.sh

@@ -6,4 +6,4 @@
 
 # Version = @@VERSION@@
 
-/usr/sbin/installer -pkg "$1/@@HOST_PKG@@.mpkg" -target /
+/usr/sbin/installer -pkg "$1/@@HOST_PKG@@.pkg" -target /