Make commands to open the ChromeVox Panel a toggle

Bug: 869898
Change-Id: I66d0ac277e0b78ea40b1eff277fbdda61bdee0fb
Reviewed-on: https://chromium-review.googlesource.com/1176423Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583461}

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

@@ -270,6 +270,12 @@ Panel.setMode = function(mode) {
  * @param {*=} opt_activateMenuTitle Title msg id of menu to open.
  */
 Panel.onOpenMenus = function(opt_event, opt_activateMenuTitle) {
+  // If the menu was already open, close it now and exit early.
+  if (Panel.mode_ != Panel.Mode.COLLAPSED) {
+    Panel.setMode(Panel.Mode.COLLAPSED);
+    return;
+  }
+
   // Eat the event so that a mousedown isn't turned into a drag, allowing
   // users to click-drag-release to select a menu item.
   if (opt_event) {