Default target_platform to None.

grit_info.py

Index: grit_info.py
diff --git a/grit_info.py b/grit_info.py
index c9ccfd1243c1adae6d5c9d0b1ccf9a2dc41a5bb8..c2fae767559ef64fef1db91f008aa675c49db8a5 100755
--- a/grit_info.py
+++ b/grit_info.py
@@ -18,7 +18,7 @@ class WrongNumberOfArguments(Exception):
   pass
 
 
-def Outputs(filename, defines, ids_file, target_platform):
+def Outputs(filename, defines, ids_file, target_platform=None):
   grd = grd_reader.Parse(
       filename, defines=defines, tags_to_ignore=set(['messages']),
       first_ids_file=ids_file, target_platform=target_platform)
@@ -62,7 +62,7 @@ def GritSourceFiles():
   return sorted(files)
 
 
-def Inputs(filename, defines, ids_file, target_platform):
+def Inputs(filename, defines, ids_file, target_platform=None):
   grd = grd_reader.Parse(
       filename, debug=False, defines=defines, tags_to_ignore=set(['message']),
       first_ids_file=ids_file, target_platform=target_platform)