Don't clobber active page scale animation with null pointer.

cc/trees/layer_tree_host_impl_unittest.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <cmath>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 1271 matching lines @@
   float min_page_scale = 0.5f;
   float max_page_scale = 4.f;
   host_impl_->sync_tree()->SetPageScaleFactorAndLimits(1.f,
                                                        min_page_scale,
                                                        max_page_scale);
   host_impl_->ActivateSyncTree();
 
   base::TimeTicks start_time = base::TimeTicks() +
                                base::TimeDelta::FromSeconds(1);
   base::TimeDelta duration = base::TimeDelta::FromMilliseconds(100);
+  base::TimeTicks third_through_animation = start_time + duration / 3;
   base::TimeTicks halfway_through_animation = start_time + duration / 2;
   base::TimeTicks end_time = start_time + duration;
   float target_scale = 2.f;
 
   scroll_layer->SetScrollOffset(gfx::ScrollOffset(50, 50));
 
   // Make sure TakePageScaleAnimation works properly.
   host_impl_->sync_tree()->SetPageScaleAnimation(
       gfx::Vector2d(),
       false,
@@ skipping 27 matching lines @@
 
   // From here on, make sure the animation runs as normal.
   did_request_redraw_ = false;
   did_request_animate_ = false;
   host_impl_->Animate(start_time);
   EXPECT_TRUE(did_request_redraw_);
   EXPECT_TRUE(did_request_animate_);
 
   did_request_redraw_ = false;
   did_request_animate_ = false;
+  host_impl_->Animate(third_through_animation);
+  EXPECT_TRUE(did_request_redraw_);
+  EXPECT_TRUE(did_request_animate_);
+
+  // Another activation shouldn't have any effect on the animation.
+  host_impl_->ActivateSyncTree();
+
+  did_request_redraw_ = false;
+  did_request_animate_ = false;
   host_impl_->Animate(halfway_through_animation);
   EXPECT_TRUE(did_request_redraw_);
   EXPECT_TRUE(did_request_animate_);
 
   did_request_redraw_ = false;
   did_request_animate_ = false;
   did_request_commit_ = false;
   host_impl_->Animate(end_time);
   EXPECT_TRUE(did_request_commit_);
   EXPECT_FALSE(did_request_animate_);
@@ skipping 5922 matching lines @@
   // surface.
   EXPECT_EQ(0, num_lost_surfaces_);
   host_impl_->DidLoseOutputSurface();
   EXPECT_EQ(1, num_lost_surfaces_);
   host_impl_->DidLoseOutputSurface();
   EXPECT_LE(1, num_lost_surfaces_);
 }
 
 }  // namespace
 }  // namespace cc