[chrome:accessibility] Only allow event recording for pages.

Using the accessibility recorder on native chrome ui crashes Chrome,
causing the bug below. This CL only shows the button to start recording
events for web pages in the chrome:accessibility page. There is also a
TODO to add support for native chrome UI later.

Bug: 1035760,785493
Change-Id: I416e00f4b8e577c5f497074243bd5eadc68ab6f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976017Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726457}

chrome/browser/resources/accessibility/accessibility.js

@@ -233,8 +233,12 @@ cr.define('accessibility', function() {
     if (hasTree) {
       row.appendChild(createHideAccessibilityTreeElement(row.id));
     }
-    row.appendChild(
-        createStartStopAccessibilityEventRecordingElement(data, row.id));
+    // The accessibility event recorder currently only works for pages.
+    // TODO(abigailbklein): Add event recording for native as well.
+    if (data.type == 'page') {
+      row.appendChild(
+          createStartStopAccessibilityEventRecordingElement(data, row.id));
+    }
 
     if (hasTree) {
       row.appendChild(createAccessibilityOutputElement(data, row.id, 'tree'));