gypd: update with required default variables.

Update the list of identity variables in gypd so that the generator will
run successfully on Chromium, which uses these variables predefined by
other generators.


Review URL:

git-svn-id: 78cadc50-ecff-11dd-a971-7dbc132099af
diff --git a/pylib/gyp/generator/ b/pylib/gyp/generator/
index 22ef57f..874b21a 100644
--- a/pylib/gyp/generator/
+++ b/pylib/gyp/generator/
@@ -39,9 +39,11 @@
 # These variables should just be spit back out as variable references.
 _generator_identity_variables = [
+  'LIB_DIR',
@@ -49,6 +51,7 @@
 # gypd doesn't define a default value for OS like many other generator