Add raw diff download link to PG diff-view

A dependent change added the ability to download a single file diff to
Gerrit. This change utilizes that new feature via a download link in the
diff-view.

Feature: Issue 4669
Change-Id: I87ef2324ff2cd7fab6eb4b2e066dd08defe7c4f0
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 a1060ab..7c5001d 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
@@ -91,6 +91,7 @@
 
     behaviors: [
       Gerrit.KeyboardShortcutBehavior,
+      Gerrit.RESTClientBehavior,
     ],
 
     observers: [
@@ -551,5 +552,11 @@
     _handleDropdownChange: function(e) {
       e.target.blur();
     },
+
+    _computeDownloadLink: function(changeNum, patchRange, path) {
+      var url = this.changeBaseURL(changeNum, patchRange.patchNum);
+      url += '/patch?zip&path=' + encodeURIComponent(path);
+      return url;
+    },
   });
 })();