Keep expanded if mouse moves off of scrollbar while dragging

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index fce0b7982d0207ca0f0cdc3b77e978f7f6b8be96..abd51c1513b5ca90b7ab9112df2f4cb19eda5ceb 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -240,7 +240,8 @@ LayerTreeHostImpl::LayerTreeHostImpl(
       id_(id),
       requires_high_res_to_draw_(false),
       is_likely_to_require_a_draw_(false),
-      mutator_(nullptr) {
+      mutator_(nullptr),
+      captured_scrollbar_layer_id_(Layer::INVALID_ID) {
   DCHECK(animation_host_);
   animation_host_->SetMutatorHostClient(this);
 
@@ -3269,6 +3270,28 @@ float LayerTreeHostImpl::DeviceSpaceDistanceToLayer(
       device_viewport_point);
 }
 
+void LayerTreeHostImpl::MouseDownAt(const gfx::Point& viewport_point) {
+  if (scroll_layer_id_when_mouse_over_scrollbar_ == Layer::INVALID_ID)
+    return;
+
+  captured_scrollbar_layer_id_ = scroll_layer_id_when_mouse_over_scrollbar_;
+  ScrollbarAnimationController* animation_controller =
+      ScrollbarAnimationControllerForId(captured_scrollbar_layer_id_);
+  if (animation_controller)
+    animation_controller->DidCaptureScrollbarBegin();
+}
+
+void LayerTreeHostImpl::MouseUp() {
+  if (captured_scrollbar_layer_id_ == Layer::INVALID_ID)
+    return;
+
+  ScrollbarAnimationController* animation_controller =
+      ScrollbarAnimationControllerForId(captured_scrollbar_layer_id_);
+  if (animation_controller)
+    animation_controller->DidCaptureScrollbarEnd();
+  captured_scrollbar_layer_id_ = Layer::INVALID_ID;
+}
+
 void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
   gfx::PointF device_viewport_point = gfx::ScalePoint(
       gfx::PointF(viewport_point), active_tree_->device_scale_factor());