cc: Make re-added animations get pushed to the compositor thread

cc/trees/layer_tree_host_unittest_animation.cc

Index: cc/trees/layer_tree_host_unittest_animation.cc
diff --git a/cc/trees/layer_tree_host_unittest_animation.cc b/cc/trees/layer_tree_host_unittest_animation.cc
index ed27a03a2f8b12bce15b9cc2814157729c267ce8..ddab640b528198dfde9ee7a34d8a253174103514 100644
--- a/cc/trees/layer_tree_host_unittest_animation.cc
+++ b/cc/trees/layer_tree_host_unittest_animation.cc
@@ -928,6 +928,58 @@ class LayerTreeHostAnimationTestAnimationsAddedToNewAndExistingLayers
 SINGLE_AND_MULTI_THREAD_BLOCKNOTIFY_TEST_F(
     LayerTreeHostAnimationTestAnimationsAddedToNewAndExistingLayers);
 
+// When a layer with an animation is removed from the tree and later re-added,
+// the animation should resume.
+class LayerTreeHostAnimationTestAnimatedLayerRemovedAndAdded
+    : public LayerTreeHostAnimationTest {
+ public:
+  LayerTreeHostAnimationTestAnimatedLayerRemovedAndAdded() {}
+
+  void SetupTree() override {
+    LayerTreeHostAnimationTest::SetupTree();
+    content_ = Layer::Create();
+    content_->SetBounds(gfx::Size(4, 4));
+    layer_tree_host()->root_layer()->AddChild(content_);
+    AddOpacityTransitionToLayer(content_.get(), 10000.0, 0.1f, 0.9f, true);
+  }
+
+  void BeginTest() override { PostSetNeedsCommitToMainThread(); }
+
+  void DidCommit() override {
+    switch (layer_tree_host()->source_frame_number()) {
+      case 1:
+        content_->RemoveFromParent();
+        break;
+      case 2:
+        layer_tree_host()->root_layer()->AddChild(content_);
+        break;
+    }
+  }
+
+  void DidActivateTreeOnThread(LayerTreeHostImpl* host_impl) override {
+    switch (host_impl->active_tree()->source_frame_number()) {
+      case 0:
+        EXPECT_TRUE(host_impl->animation_registrar()->needs_animate_layers());
+        break;
+      case 1:
+        EXPECT_FALSE(host_impl->animation_registrar()->needs_animate_layers());
+        break;
+      case 2:
+        EXPECT_TRUE(host_impl->animation_registrar()->needs_animate_layers());
+        EndTest();
+        break;
+    }
+  }
+
+  void AfterTest() override {}
+
+ private:
+  scoped_refptr<Layer> content_;
+};
+
+SINGLE_AND_MULTI_THREAD_TEST_F(
+    LayerTreeHostAnimationTestAnimatedLayerRemovedAndAdded);
+
 class LayerTreeHostAnimationTestAddAnimationAfterAnimating
     : public LayerTreeHostAnimationTest {
  public: