[breadcrumb] Add unittest support for <cr-action-menu>

<cr-action-menu> is Polymer3 and so won't be active during a files app
unittest framework.

Add window.UNIT_TEST guards to prevent the breadcrumb.js unittest from
trying to access undefined (in unittest) <cr-action-menu> api.

Define window.UNIT_TEST and window.IN_TEST for closure compilation. It
seems window.IN_TEST is only closure defined in gallery externs so add
these new closure defines to a files app common externs file.

Bug: 1061388
Change-Id: Id5c11649f32969d5b1b11cbf30236c80edbc69c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138939Reviewed-by: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756983}

ui/file_manager/externs/app_window_common.js

@@ -30,3 +30,17 @@ Window.prototype.HTMLImports;
  * @type {function(function())}
  */
 Window.prototype.HTMLImports.whenReady;
+
+/**
+ * True if in test: set by the background.js page.
+ *
+ * @type {boolean}
+ */
+Window.prototype.IN_TEST;
+
+/**
+ * Set true in some unit tests.
+ *
+ * @type {boolean}
+ */
+Window.prototype.UNIT_TEST;

ui/file_manager/file_manager/foreground/js/ui/breadcrumb.js

@@ -423,10 +423,10 @@ class BreadCrumb extends HTMLElement {
     // Show drop-down below the elider button.
     const menu = this.shadowRoot.querySelector('cr-action-menu');
     const top = elider.offsetTop + elider.offsetHeight + 4;
-    menu.showAt(elider, {top: top});
+    !window.UNIT_TEST && menu.showAt(elider, {top: top});
 
     // Style drop-down and horizontal position.
-    const dialog = menu.getDialog();
+    const dialog = !window.UNIT_TEST ? menu.getDialog() : {style: {}};
     dialog.style['left'] = position + 'px';
     dialog.style['right'] = position + 'px';
     dialog.style['overflow'] = 'hidden auto';
@@ -455,7 +455,7 @@ class BreadCrumb extends HTMLElement {
 
     // Close the drop-down <dialog> if needed.
     const menu = this.shadowRoot.querySelector('cr-action-menu');
-    if (menu.getDialog().hasAttribute('open')) {
+    if (!window.UNIT_TEST && menu.getDialog().hasAttribute('open')) {
       menu.close();
     }
   }