Add in-profile external_extensions.json file for Chrome OS OEM customization.

chrome/common/chrome_paths.h

Index: chrome/common/chrome_paths.h
diff --git a/chrome/common/chrome_paths.h b/chrome/common/chrome_paths.h
index 09fe1b69eed44fca58607f8443ed762021b166a9..0626fc26059ed578db63a84d7dd7b4e138966de0 100644
--- a/chrome/common/chrome_paths.h
+++ b/chrome/common/chrome_paths.h
@@ -49,6 +49,12 @@ enum {
   DIR_MANAGED_PREFS,            // Directory that stores the managed prefs plist
                                 // files for the current user.
 #endif
+#if defined(OS_CHROMEOS)
+  DIR_USER_EXTERNAL_EXTENSIONS,  // Directory for per-user external extensions.
+                                 // Used for OEM customization on Chrome OS.
+                                 // Getting this path does not create it.
+#endif
+
   FILE_RESOURCE_MODULE,         // Full path and filename of the module that
                                 // contains embedded resources (version,
                                 // strings, images, etc.).