Make CreateDirectory consistent on Win and Posix when directory exists.

base/file_util.h

Index: base/file_util.h
===================================================================
--- base/file_util.h	(revision 1925)
+++ base/file_util.h	(working copy)
@@ -222,7 +222,8 @@
                             std::wstring* new_temp_path);
 
 // Creates a directory, as well as creating any parent directories, if they
-// don't exist. Returns 'true' on successful creation.
+// don't exist. Returns 'true' on successful creation, or if the directory
+// already exists.
 bool CreateDirectory(const std::wstring& full_path);
 
 // Returns the file size. Returns true on success.
@@ -320,4 +321,3 @@
 }  // namespace file_util
 
 #endif  // BASE_FILE_UTIL_H_
-