GTK: Turn on the GTK+ integration by default.

chrome/test/data/profiles/custom_frame/Default/Preferences

 {
+   "extensions": {
+      "theme": {
+          "use_system": false
+      }
+   },
    "browser": {
       "custom_chrome_frame": true
    },
    "profile": {
       "exited_cleanly": true,
       "id": "not-signed-in",
       "name": "Not Signed In",
       "nickname": "not-signed-in"
    },
    "spellcheck": {
       "dictionary": "en-US"
    }
 }