Fix NaCl Gyp error: fix building with nacl_disabled=1

https://codereview.chromium.org/92043002 caused nacl_switches_64
not to get built if nacl_disabled==1, but things expect it to be there
regardless. Add a dummy target to build if nacl_disabled == 1 to
keep gyp happy.

Specifically, common_constants_win64 depends on nacl_switches_win64.

BUG=244791
NOTRY=true

Review URL: https://codereview.chromium.org/95003006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237802 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237802