Add GrDiffCursor.moveToLastChunk

The new method is analogous to GrDiffCursor.moveToFirstChunk, and can be used
when navigating backwards through diffs.

Change-Id: I0c53c3247d4b996fe0dcc0df65e5497494d7c9e7

polygerrit-ui/app/elements/shared/gr-cursor-manager/gr-cursor-manager.js

@@ -163,6 +163,12 @@
       }
     }
 
+    moveToEnd() {
+      if (this.stops.length) {
+        this.setCursor(this.stops[this.stops.length - 1]);
+      }
+    }
+
     setCursorAtIndex(index, opt_noScroll) {
       this.setCursor(this.stops[index], opt_noScroll);
     }