Convert windows path seperators to unix like while doing IncludeWhatYouUse.

Review URL: https://codereview.appspot.com/5601049

cpplint/cpplint.py

@@ -3054,7 +3054,7 @@ def CheckForIncludeWhatYouUse(filename, clean_lines, include_state, error,
   header_found = False
 
   # Use the absolute path so that matching works properly.
-  abs_filename = os.path.abspath(filename)
+  abs_filename = FileInfo(filename).FullName()
 
   # For Emacs's flymake.
   # If cpplint is invoked from Emacs's flymake, a temporary file is generated