Add external extensions json source in proper mac location.

chrome/common/chrome_paths_mac.mm

Index: chrome/common/chrome_paths_mac.mm
diff --git a/chrome/common/chrome_paths_mac.mm b/chrome/common/chrome_paths_mac.mm
index 7331d78330922a21308be230ea632ca62286aecf..9df7cd5a8a1acce5e2053b755086fdc90d064397 100644
--- a/chrome/common/chrome_paths_mac.mm
+++ b/chrome/common/chrome_paths_mac.mm
@@ -105,6 +105,10 @@ bool GetUserDocumentsDirectory(FilePath* result) {
   return base::mac::GetUserDirectory(NSDocumentDirectory, result);
 }
 
+bool GetGlobalApplicationSupportDirectory(FilePath* result) {
+  return base::mac::GetLocalDirectory(NSApplicationSupportDirectory, result);
+}
+
 void GetUserCacheDirectory(const FilePath& profile_dir, FilePath* result) {
   // If the profile directory is under ~/Library/Application Support,
   // use a suitable cache directory under ~/Library/Caches.  For