Fire diff-view title-change event in an async

Formerly, the title-change event was fired too early, and failed to
set the actual title.

Change-Id: Ia32e76023c426cf6b8a3ad1456490885fb22bd67
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 7181270..f885c25 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
@@ -461,8 +461,10 @@
       };
       this._path = value.path;
 
-      this.fire('title-change',
-          {title: this._computeTruncatedFileDisplayName(this._path)});
+      this.async(() => {
+        this.fire('title-change',
+            {title: this._computeTruncatedFileDisplayName(this._path)});
+      });
 
       // When navigating away from the page, there is a possibility that the
       // patch number is no longer a part of the URL (say when navigating to