DevTools: Add links to the storage docs

This CL adds links from the storage panes to the official DevTools
storage documentation by means of a new `appendLink()` method on
`EmptyWidget`.

I added the `appendLink()` method because I plan on repeating this
pattern as much as possible wherever `EmptyWidget` is used.
`EmptyWidget` is a good surface for linking out to the documentation
because it's often the first thing that users see while exploring the
DevTools UI.

All of the panes on the Application panel reuse a single `EmptyWidget`
instance so I had to add some logic to conditionally display the link.

Bug: 956639
Change-Id: I5ee17c5f10ef85b8ae2957c93ee7d0d7edf32e81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588758
Reviewed-by: Dmitry Gozman <[email protected]>
Commit-Queue: Kayce Basques <[email protected]>
Auto-Submit: Kayce Basques <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#655288}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ef2f2f24b6fee8e721a53adb6e244133645f2821
diff --git a/front_end/ui/EmptyWidget.js b/front_end/ui/EmptyWidget.js
index c35c254..9ac0493 100644
--- a/front_end/ui/EmptyWidget.js
+++ b/front_end/ui/EmptyWidget.js
@@ -52,6 +52,14 @@
   }
 
   /**
+   * @param {string} link
+   * @return {!Node}
+   */
+  appendLink(link) {
+    return this._contentElement.appendChild(UI.XLink.create(link, 'Learn more'));
+  }
+
+  /**
    * @param {string} text
    */
   set text(text) {