Fix usage of property '_projectConfig'

- gr-change-view: access with wrong name
- gr-diff-view: missing property declaration

Change-Id: I6751e16d80698fa86029d93a45ee54fc5a7b9b5d
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
index 17aa0c4..85cf380 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
@@ -69,6 +69,7 @@
         value: true,
       },
       _prefs: Object,
+      _projectConfig: Object,
       _userPrefs: Object,
       _diffMode: {
         type: String,