This CL adds support for extension in GYP.

pylib/gyp/xcodeproj_file.py

Index: pylib/gyp/xcodeproj_file.py
diff --git a/pylib/gyp/xcodeproj_file.py b/pylib/gyp/xcodeproj_file.py
index 79c3abcf025604f4127c7a31b9b89ef05f8a3b0e..b8bb39a2de915ca7e69bb3f44aca559557a70305 100644
--- a/pylib/gyp/xcodeproj_file.py
+++ b/pylib/gyp/xcodeproj_file.py
@@ -2237,10 +2237,12 @@ class PBXNativeTarget(XCTarget):
   # Mapping from Xcode product-types to settings.  The settings are:
   #  filetype : used for explicitFileType in the project file
   #  prefix : the prefix for the file name
-  #  suffix : the suffix for the filen ame
+  #  suffix : the suffix for the file name
   _product_filetypes = {
     'com.apple.product-type.application':       ['wrapper.application',
                                                  '', '.app'],
+    'com.apple.product-type.app-extension':     ['wrapper.application',
+                                                 '', '.appex'],
     'com.apple.product-type.bundle':            ['wrapper.cfbundle',
                                                  '', '.bundle'],
     'com.apple.product-type.framework':         ['wrapper.framework',