Update Dr. Memory bot factories to fully handle a 3rd platform, MacOS.

masters/master.client.drmemory/master.cfg

Index: masters/master.client.drmemory/master.cfg
===================================================================
--- masters/master.client.drmemory/master.cfg	(revision 293033)
+++ masters/master.client.drmemory/master.cfg	(working copy)
@@ -152,10 +152,10 @@
 # Bots that run on every commit: the pre-commit suite and the packager.
 for (os, os_versions) in MAINBOTS:
   c['builders'].append({'name': '%s-builder' % os,
-                        'factory': F_DRMPACK(os == "win")})
+                        'factory': F_DRMPACK(os)})
   for version in os_versions:
     c['builders'].append({'name': '%s-%s-drm' % (os, version),
-                          'factory': F_DRM(os == 'win')})
+                          'factory': F_DRM(os)})
 
 # Miscellaneous stability bots that run periodically.
 c['builders'].append({'name': 'win7-cr-builder',