Use gn_helpers to deserialize GN lists.

build/gn_helpers.py

Index: build/gn_helpers.py
diff --git a/build/gn_helpers.py b/build/gn_helpers.py
index 07a7c3e913c1ebeaa9936df9f5fd7f53af62ffe9..dc0820a62bcb12d3e3c131f463bc65893c95f58e 100644
--- a/build/gn_helpers.py
+++ b/build/gn_helpers.py
@@ -5,7 +5,19 @@
 """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