CC Animation: Activate/deactivate ElementAnimations on Layer addition/removal.

This is a CC-side fix which can be useful for all CC clients.

Blink side: We will need to fix the race in CompositedLayerMapping destruction vs. CompositorAnimationPlayer detachment separately.

BUG=615471
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2042713002
Cr-Commit-Position: refs/heads/master@{#398209}

cc/animation/element_animations.cc

@@ -89,6 +89,9 @@ void ElementAnimations::ElementRegistered(ElementId element_id,
                                           ElementListType list_type) {
   DCHECK_EQ(element_id_, element_id);
 
+  if (!has_element_in_any_list())
+    UpdateActivation(FORCE_ACTIVATION);
+
   if (list_type == ElementListType::ACTIVE)
     set_has_element_in_active_list(true);
   else
@@ -102,6 +105,9 @@ void ElementAnimations::ElementUnregistered(ElementId element_id,
     set_has_element_in_active_list(false);
   else
     set_has_element_in_pending_list(false);
+
+  if (!has_element_in_any_list())
+    animation_host_->DidDeactivateElementAnimations(this);
 }
 
 void ElementAnimations::AddPlayer(AnimationPlayer* player) {

cc/animation/element_animations.h

@@ -125,6 +125,9 @@ class CC_EXPORT ElementAnimations : public base::RefCounted<ElementAnimations> {
   bool has_element_in_pending_list() const {
     return has_element_in_pending_list_;
   }
+  bool has_element_in_any_list() const {
+    return has_element_in_active_list_ || has_element_in_pending_list_;
+  }
 
   void set_has_element_in_active_list(bool has_element_in_active_list) {
     has_element_in_active_list_ = has_element_in_active_list;

cc/trees/layer_tree_host_impl_unittest.cc

@@ -1453,6 +1453,60 @@ TEST_F(LayerTreeHostImplTest, AnimationSchedulingCommitToActiveTree) {
   host_impl_ = nullptr;
 }
 
+TEST_F(LayerTreeHostImplTest, AnimationSchedulingOnLayerDestruction) {
+  host_impl_->SetViewportSize(gfx::Size(50, 50));
+
+  host_impl_->active_tree()->SetRootLayer(
+      LayerImpl::Create(host_impl_->active_tree(), 1));
+  LayerImpl* root = host_impl_->active_tree()->root_layer();
+  root->SetBounds(gfx::Size(50, 50));
+
+  root->AddChild(LayerImpl::Create(host_impl_->active_tree(), 2));
+  LayerImpl* child = root->children()[0];
+  child->SetBounds(gfx::Size(10, 10));
+  child->draw_properties().visible_layer_rect = gfx::Rect(10, 10);
+  child->SetDrawsContent(true);
+
+  // Add a translate animation.
+  TransformOperations start;
+  start.AppendTranslate(6.f, 7.f, 0.f);
+  TransformOperations end;
+  end.AppendTranslate(8.f, 9.f, 0.f);
+  AddAnimatedTransformToLayerWithPlayer(child->id(), timeline(), 4.0, start,
+                                        end);
+
+  base::TimeTicks now = base::TimeTicks::Now();
+  host_impl_->WillBeginImplFrame(
+      CreateBeginFrameArgsForTesting(BEGINFRAME_FROM_HERE, now));
+  EXPECT_TRUE(did_request_next_frame_);
+  did_request_next_frame_ = false;
+
+  host_impl_->ActivateAnimations();
+  // On activating an animation, we should request another frame so that we'll
+  // continue ticking the animation.
+  EXPECT_TRUE(did_request_next_frame_);
+  did_request_next_frame_ = false;
+
+  // The next frame after activating, we'll tick the animation again.
+  host_impl_->Animate();
+  // An animation exists on the active layer. Doing Animate() requests another
+  // frame after the current one.
+  EXPECT_TRUE(did_request_next_frame_);
+  did_request_next_frame_ = false;
+
+  // Destroy layer, unregister animation target (element).
+  child->SetParent(nullptr);
+  root->RemoveChildForTesting(child);
+  child = nullptr;
+
+  // Doing Animate() doesn't request another frame after the current one.
+  host_impl_->Animate();
+  EXPECT_FALSE(did_request_next_frame_);
+
+  host_impl_->Animate();
+  EXPECT_FALSE(did_request_next_frame_);
+}
+
 class MissingTilesLayer : public LayerImpl {
  public:
   MissingTilesLayer(LayerTreeImpl* layer_tree_impl, int id)

cc/trees/layer_tree_host_unittest_animation.cc

@@ -1250,7 +1250,7 @@ class LayerTreeHostAnimationTestAnimatedLayerRemovedAndAdded
             player_->element_animations()->has_element_in_active_list());
         EXPECT_FALSE(
             player_->element_animations()->has_element_in_pending_list());
-        EXPECT_TRUE(layer_tree_host()->animation_host()->NeedsAnimateLayers());
+        EXPECT_FALSE(layer_tree_host()->animation_host()->NeedsAnimateLayers());
         break;
       case 2:
         layer_tree_host()->root_layer()->AddChild(layer_);
@@ -1278,7 +1278,7 @@ class LayerTreeHostAnimationTestAnimatedLayerRemovedAndAdded
       case 1:
         EXPECT_FALSE(
             player_impl->element_animations()->has_element_in_active_list());
-        EXPECT_TRUE(host_impl->animation_host()->NeedsAnimateLayers());
+        EXPECT_FALSE(host_impl->animation_host()->NeedsAnimateLayers());
         break;
       case 2:
         EXPECT_TRUE(