Proactively clear ChromeVox focus ring

Change-Id: I5712c277c3a52b824e35ebc990e78e2131b18a73
Reviewed-on: https://chromium-review.googlesource.com/1217031Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590412}

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

@@ -178,15 +178,21 @@ Background.prototype = {
     // the user navigates.
     cvox.ChromeVox.braille.thaw();
 
-    if (newRange && !newRange.isValid())
+    if (newRange && !newRange.isValid()) {
+      chrome.accessibilityPrivate.setFocusRing([]);
       return;
+    }
 
     this.currentRange_ = newRange;
     ChromeVoxState.observers.forEach(function(observer) {
       observer.onCurrentRangeChanged(newRange);
     });
 
-    if (this.currentRange_) {
+    if (!this.currentRange_) {
+      chrome.accessibilityPrivate.setFocusRing([]);
+      return;
+    }
+
     var start = this.currentRange_.start.node;
     start.makeVisible();
 
@@ -201,7 +207,6 @@ Background.prototype = {
     var url = root.docUrl;
     url = url.substring(0, url.indexOf('#')) || url;
     cvox.ChromeVox.position[url] = position;
-    }
   },
 
   /**