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 36607354a0fdac4acf0e775d41c133fe15693dec..05bca612c5e074cdb90a284f098ea2d502d7d174 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -209,6 +209,7 @@ LayerTreeHostImpl::LayerTreeHostImpl(
       wheel_scrolling_(false),
       scroll_affects_scroll_handler_(false),
       scroll_layer_id_when_mouse_over_scrollbar_(Layer::INVALID_ID),
+      captured_scrollbar_layer_id_(Layer::INVALID_ID),
       tile_priorities_dirty_(false),
       settings_(settings),
       visible_(false),
@@ -3228,6 +3229,28 @@ float LayerTreeHostImpl::DeviceSpaceDistanceToLayer(
       device_viewport_point);
 }
 
+void LayerTreeHostImpl::MouseDown() {
+  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());