Fixing focus on diff preferences

The auto focus was broken when diff preferencies diff were open

Change-Id: Icaf4c86dc3405cce390570f8c1a38e4a3f317845

polygerrit-ui/app/elements/diff/gr-diff-preferences-dialog/gr-diff-preferences-dialog.js

@@ -34,13 +34,13 @@
 
     getFocusStops() {
       return {
-        start: this.$.contextSelect,
+        start: this.$.diffPreferences.$.contextSelect,
         end: this.$.saveButton,
       };
     },
 
     resetFocus() {
-      this.$.contextSelect.focus();
+      this.$.diffPreferences.$.contextSelect.focus();
     },
 
     _computeHeaderClass(changed) {

polygerrit-ui/app/elements/shared/gr-select/gr-select.js

@@ -60,6 +60,10 @@
       this.bindValue = this.nativeSelect.value;
     },
 
+    focus() {
+      this.nativeSelect.focus();
+    },
+
     ready() {
       // If not set via the property, set bind-value to the element value.
       if (this.bindValue == undefined) {