Adds ability to suspend notifying observers when display list is changed

ui/display/display_list_unittest.cc

Index: ui/display/display_list_unittest.cc
diff --git a/ui/display/display_list_unittest.cc b/ui/display/display_list_unittest.cc
index 9873450bc0ad6fd06728f18047422ec3235e613d..49543db1210afd30e63a7bec3416fef3117acdd2 100644
--- a/ui/display/display_list_unittest.cc
+++ b/ui/display/display_list_unittest.cc
@@ -109,5 +109,31 @@ TEST(DisplayListTest, AddUpdateRemove) {
   EXPECT_EQ(3, display_list.GetPrimaryDisplayIterator()->id());
 }
 
+TEST(DisplayListTest, SuspendUpdates) {
+  DisplayList display_list;
+  display_list.AddDisplay(display::Display(2, gfx::Rect(0, 0, 801, 802)),
+                          DisplayList::Type::PRIMARY);
+  DisplayObserverImpl observer;
+  display_list.AddObserver(&observer);
+  {
+    // Suspend updates and add a new display.
+    std::unique_ptr<DisplayListObserverLock> lock =
+        display_list.SuspendObserverUpdates();
+    display_list.AddDisplay(display::Display(3, gfx::Rect(0, 0, 809, 802)),
+                            DisplayList::Type::NOT_PRIMARY);
+    EXPECT_EQ(2u, display_list.displays().size());
+    // No update should have been generated.
+    EXPECT_TRUE(observer.GetAndClearChanges().empty());
+  }
+  // The lock has been destroyed, but no updates should be sent yet.
+  EXPECT_TRUE(observer.GetAndClearChanges().empty());
+
+  // Update a display and verify observer called.
+  display::Display updated_display = display_list.displays()[0];
+  updated_display.set_bounds(gfx::Rect(0, 0, 803, 802));
+  display_list.UpdateDisplay(updated_display, DisplayList::Type::PRIMARY);
+  EXPECT_EQ("Changed id=2 bounds", observer.GetAndClearChanges());
+}
+
 }  // namespace
 }  // namespace display