Support GYP_CROSSCOMPILE variable in gyp_helper.

Support setting GYP_CROSSCOMPILE from gyp_env files.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/14652028

git-svn-id: http://src.chromium.org/svn/trunk/src/build@198797 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

gyp_helper.py

@@ -31,6 +31,7 @@ def apply_gyp_environment_from_file(file_path):
       'CXX_wrapper',
       'GYP_DEFINES',
       'GYP_GENERATOR_FLAGS',
+      'GYP_CROSSCOMPILE',
       'GYP_GENERATOR_OUTPUT',
       'GYP_GENERATORS',
   )