fixes focus recovery when closing a window

Scenario:
- chrome://settings
- under advanced -> accessibility, click add accessibility features
- Ctrl+w
- Search+Right

result:
ChromeVox accessibility focus moves to the root of the settings page

Explanation

The sequence of events immediately after Ctrl+w are as follows:
"focus webView"
...
"ariaAttributeChanged window"
"blur webView"
"focus unknown"
...
"ariaAttributeChanged window"
"blur unknown"
"focus link"
...
"<accessibility event type> <target node's role>"

The typical event stream from an interaction where a page gets initial focus
always involves the "rootWebArea" getting focused (this is typically a child of
the webView node).

Fix

The fix is to always refresh DesktopAutomationHandler's |lastRootUrl| state for
every focus event not just ones on a rootWebArea.

This state is used to prevent ChromeVox from trying to recover focus when a user
has already started navigating into a page before the page has completely
loaded. In the case where someone closes a window, the focus never actually
lands on the rootWebArea first but instead on the webView only.

The existing check is:
- ChromeVox accessibility focus's root == the new focus's root
and
- the last root url == the new focus's root url

Test:
The cases that continue to work:
- alt+tab (roots are different, root urls are different, focus recovery triggers)
- reload (ctrl+r)
- ctrl+tab
- alt+left (root can actually be the same, but root urls different which triggers recovery)

New:
- Ctrl+w

Verify all these cases recover to the previous place and Search+left/right work
as expected after.

Bug: 1003992

Change-Id: If09ab14a4aafb3e0a849266eebf69315b277d005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804048
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696605}

chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js

@@ -422,6 +422,9 @@ DesktopAutomationHandler.prototype = {
     if (!node.root)
       return;
 
+    // Update the focused root url, which gets used as part of focus recovery.
+    this.lastRootUrl_ = node.root.docUrl || '';
+
     var event = new CustomAutomationEvent(EventType.FOCUS, node, evt.eventFrom);
     this.onEventDefault(event);