CC Animations: Fix LayerTreeHostTimelinesTestSetPotentiallyAnimatingOnLacDestruction flakiness.

Check active tree state separately, on each PrepareToDrawOnThread.

The source of the flakiness that the
test previously made assumptions about the timing of draws (when the active
tree's draw properties get updated) vs commits.

BUG=546867
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1472723002

Cr-Commit-Position: refs/heads/master@{#361237}

cc/trees/layer_tree_host_unittest_animation_timelines.cc

@@ -1004,6 +1004,9 @@ class LayerTreeHostTimelinesTestSetPotentiallyAnimatingOnLacDestruction
     : public LayerTreeHostTimelinesTest {
  public:
   void SetupTree() override {
+    prev_screen_space_transform_is_animating_ = true;
+    screen_space_transform_animation_stopped_ = false;
+
     LayerTreeHostTimelinesTest::SetupTree();
     AttachPlayersToTimeline();
     player_->AttachLayer(layer_tree_host()->root_layer()->id());
@@ -1013,24 +1016,11 @@ class LayerTreeHostTimelinesTestSetPotentiallyAnimatingOnLacDestruction
   void BeginTest() override { PostSetNeedsCommitToMainThread(); }
 
   void CommitCompleteOnThread(LayerTreeHostImpl* host_impl) override {
-    if (host_impl->active_tree()->source_frame_number() == 1) {
-      EXPECT_TRUE(host_impl->active_tree());
-      EXPECT_TRUE(host_impl->active_tree()->root_layer());
-      EXPECT_TRUE(host_impl->pending_tree());
-      EXPECT_TRUE(host_impl->pending_tree()->root_layer());
-
-      EXPECT_EQ(2, host_impl->pending_tree()->source_frame_number());
-
-      EXPECT_TRUE(host_impl->active_tree()
+    if (host_impl->pending_tree()->source_frame_number() <= 1) {
+      EXPECT_TRUE(host_impl->pending_tree()
                       ->root_layer()
                       ->screen_space_transform_is_animating());
-      EXPECT_FALSE(host_impl->pending_tree()
-                       ->root_layer()
-                       ->screen_space_transform_is_animating());
-    } else if (host_impl->active_tree()->source_frame_number() == 2) {
-      EXPECT_FALSE(host_impl->active_tree()
-                       ->root_layer()
-                       ->screen_space_transform_is_animating());
+    } else {
       EXPECT_FALSE(host_impl->pending_tree()
                        ->root_layer()
                        ->screen_space_transform_is_animating());
@@ -1047,12 +1037,44 @@ class LayerTreeHostTimelinesTestSetPotentiallyAnimatingOnLacDestruction
     }
   }
 
+  DrawResult PrepareToDrawOnThread(LayerTreeHostImpl* host_impl,
+                                   LayerTreeHostImpl::FrameData* frame_data,
+                                   DrawResult draw_result) override {
+    const bool screen_space_transform_is_animating =
+        host_impl->active_tree()
+            ->root_layer()
+            ->screen_space_transform_is_animating();
+
+    // Check that screen_space_transform_is_animating changes only once.
+    if (screen_space_transform_is_animating &&
+        prev_screen_space_transform_is_animating_)
+      EXPECT_FALSE(screen_space_transform_animation_stopped_);
+    if (!screen_space_transform_is_animating &&
+        prev_screen_space_transform_is_animating_) {
+      EXPECT_FALSE(screen_space_transform_animation_stopped_);
+      screen_space_transform_animation_stopped_ = true;
+    }
+    if (!screen_space_transform_is_animating &&
+        !prev_screen_space_transform_is_animating_)
+      EXPECT_TRUE(screen_space_transform_animation_stopped_);
+
+    prev_screen_space_transform_is_animating_ =
+        screen_space_transform_is_animating;
+
+    return draw_result;
+  }
+
   void DrawLayersOnThread(LayerTreeHostImpl* host_impl) override {
     if (host_impl->active_tree()->source_frame_number() >= 2)
       EndTest();
   }
 
-  void AfterTest() override {}
+  void AfterTest() override {
+    EXPECT_TRUE(screen_space_transform_animation_stopped_);
+  }
+
+  bool prev_screen_space_transform_is_animating_;
+  bool screen_space_transform_animation_stopped_;
 };
 
 MULTI_THREAD_TEST_F(