WebUI: fix flaky cr-action-menu test.

Bug: 812947
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: If2a653a7784b464d44bbde72d42df0ca719c2e90
Reviewed-on: https://chromium-review.googlesource.com/923580Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537477}

chrome/test/data/webui/cr_elements/cr_action_menu_test.js

@@ -306,7 +306,9 @@ suite('CrActionMenu', function() {
   });
 
 
-  test('[auto-reposition] enables repositioning if content changes', function() {
+  test(
+      '[auto-reposition] enables repositioning if content changes',
+      function(done) {
         menu.autoReposition = true;
 
         dots.style.marginLeft = '800px';
@@ -325,10 +327,7 @@ suite('CrActionMenu', function() {
         const lastMenuLeft = menuRect.left;
         const lastMenuWidth = menuRect.width;
 
-    // Still anchored at the right place after content size changes.
-    items[0].textContent = 'this is a long string to make menu wide';
-    // Flush to wait for resizeObeserver's cycle.
-    return PolymerTest.flushTasks().then(() => {
+        menu.addEventListener('cr-action-menu-repositioned', () => {
           assertTrue(menu.open);
           menuRect = menu.getBoundingClientRect();
           // Test that menu width got larger.
@@ -340,7 +339,11 @@ suite('CrActionMenu', function() {
               Math.round(dotsRect.left + dotsRect.width),
               Math.round(menuRect.left + menuRect.width));
           assertEquals(dotsRect.top, menuRect.top);
+          done();
         });
+
+        // Still anchored at the right place after content size changes.
+        items[0].textContent = 'this is a long string to make menu wide';
       });
 
   suite('offscreen scroll positioning', function() {

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js

@@ -440,8 +440,10 @@ Polymer({
 
     if (this.autoReposition) {
       this.resizeObserver_ = new ResizeObserver(() => {
-        if (this.lastConfig_)
+        if (this.lastConfig_) {
           this.positionDialog_(this.lastConfig_);
+          this.fire('cr-action-menu-repositioned');  // For easier testing.
+        }
       });
 
       this.resizeObserver_.observe(this);