Fix NSString conversions to treat a null NSString as a string of length 0, in...

base/sys_string_conversions.h

Index: base/sys_string_conversions.h
===================================================================
--- base/sys_string_conversions.h	(revision 31117)
+++ base/sys_string_conversions.h	(working copy)
@@ -71,7 +71,8 @@
 string16 SysCFStringRefToUTF16(CFStringRef ref);
 std::wstring SysCFStringRefToWide(CFStringRef ref);
 
-// Same, but accepts NSString input.
+// Same, but accepts NSString input. Converts nil NSString* to the appropriate
+// string type of length 0.
 std::string SysNSStringToUTF8(NSString* ref);
 string16 SysNSStringToUTF16(NSString* ref);
 std::wstring SysNSStringToWide(NSString* ref);