[DevTools] Add a `Learn More` link to Background Service panes.

Screenshot: https://bugs.chromium.org/p/chromium/issues/detail?id=942174#c40
Bug: 942174
Change-Id: I5d8b275671b78de6b96aa5f4704a2210bc1bd75f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760738
Commit-Queue: Rayan Kanso <[email protected]>
Reviewed-by: Alexei Filippov <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#692458}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e61293ea18d25fb059dc474444cf704409ecc73a
diff --git a/front_end/resources/BackgroundServiceView.js b/front_end/resources/BackgroundServiceView.js
index d64cfe4..fec6f91 100644
--- a/front_end/resources/BackgroundServiceView.js
+++ b/front_end/resources/BackgroundServiceView.js
@@ -266,6 +266,33 @@
   }
 
   /**
+   * @return {!Element}
+   */
+  _createLearnMoreLink() {
+    let url =
+        'https://developers.google.com/web/tools/chrome-devtools/javascript/background-services?utm_source=devtools';
+
+    switch (this._serviceName) {
+      case Protocol.BackgroundService.ServiceName.BackgroundFetch:
+        url += '#fetch';
+        break;
+      case Protocol.BackgroundService.ServiceName.BackgroundSync:
+        url += '#sync';
+        break;
+      case Protocol.BackgroundService.ServiceName.PushMessaging:
+        url += '#push';
+        break;
+      case Protocol.BackgroundService.ServiceName.Notifications:
+        url += '#notifications';
+        break;
+      default:
+        break;
+    }
+
+    return UI.XLink.create(url, ls`Learn more`);
+  }
+
+  /**
    * @param {?Resources.BackgroundServiceView.EventDataNode} dataNode
    */
   _showPreview(dataNode) {
@@ -307,6 +334,8 @@
       inlineButton.classList.add('background-service-record-inline-button');
       centered.createChild('p').appendChild(
           UI.formatLocalized('Click the record button %s or hit %s to start recording.', [inlineButton, recordKey]));
+
+      centered.appendChild(this._createLearnMoreLink());
     }
 
     this._preview.show(this._previewPanel.contentElement);
diff --git a/front_end/resources/backgroundServiceView.css b/front_end/resources/backgroundServiceView.css
index 0fd3c0c..6a52e00 100644
--- a/front_end/resources/backgroundServiceView.css
+++ b/front_end/resources/backgroundServiceView.css
@@ -21,12 +21,12 @@
 .background-service-preview > div {
     max-width: 450px;
     margin: 10px;
+    text-align: center;
 }
 
 .background-service-preview > div > p {
     flex: none;
     white-space: pre-line;
-    text-align: center;
 }
 
 .background-service-shortcut {