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..3261fd391dc002386987b933c61253d91f747d25 100644
--- a/ui/display/display_list_unittest.cc
+++ b/ui/display/display_list_unittest.cc
@@ -109,5 +109,50 @@ 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 and should now notify observers.
+  EXPECT_EQ("Added id=3", observer.GetAndClearChanges());
+
+  // Two locks with an add followed by an update.
+  {
+    std::unique_ptr<DisplayListObserverLock> lock1 =
+        display_list.SuspendObserverUpdates();
+    {
+      std::unique_ptr<DisplayListObserverLock> lock2 =
+          display_list.SuspendObserverUpdates();
+      display_list.AddDisplay(display::Display(4, gfx::Rect(0, 0, 809, 802)),
+                              DisplayList::Type::NOT_PRIMARY);
+      EXPECT_TRUE(observer.GetAndClearChanges().empty());
+      EXPECT_EQ(3u, display_list.displays().size());
+    }
+    // No updates should have been generated as there is still a valid lock.
+    EXPECT_TRUE(observer.GetAndClearChanges().empty());
+
+    // Update the display. Lock still held, so no updates.
+    display::Display updated_display = display_list.displays()[2];
+    updated_display.set_bounds(gfx::Rect(0, 0, 803, 802));
+    display_list.UpdateDisplay(updated_display, DisplayList::Type::NOT_PRIMARY);
+    EXPECT_TRUE(observer.GetAndClearChanges().empty());
+  }
+  // All locks have been destroyed, so observer should be called.
+  EXPECT_EQ("Added id=4", observer.GetAndClearChanges());
+}
+
 }  // namespace
 }  // namespace display