Use gn_helpers to deserialize GN lists.

build/gn_helpers.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Helper functions useful when writing scripts that integrate with GN.
 
 The main functions are ToGNString and FromGNString which convert between
-serialized GN veriables and Python variables."""
+serialized GN veriables and Python variables.
+
+To use in a random python file in the build:
+
+  import os
+  import sys
+
+  sys.path.append(os.path.join(os.path.dirname(__file__),
+                               os.pardir, os.pardir, "build"))
+  import gn_helpers
+
+Where the sequence of parameters to join is the relative path from your source
+file to the build directory."""
 
 class GNException(Exception):
   pass
 
 
 def ToGNString(value, allow_dicts = True):
   """Prints the given value to stdout.
 
   allow_dicts indicates if this function will allow converting dictionaries
   to GN scopes. This is only possible at the top level, you can't nest a
@@ skipping 248 matching lines @@
     location in the input. If it does, the text is consumed and the function
     returns true. Otherwise, returns false and the current position is
     unchanged."""
     end = self.cur + len(constant)
     if end >= len(self.input):
       return False  # Not enough room.
     if self.input[self.cur:end] == constant:
       self.cur = end
       return True
     return False