Merge fixup. Note saved original in save.

Author: sherm1

multibody/tree/multibody_tree.cc

@@ -792,8 +792,14 @@ void MultibodyTree<T>::CreateJointImplementations() {
     Mobilizer<T>* mobilizer = owned_mobilizer.get();
     mobilizer->set_model_instance(joint.model_instance());
     mobilizer->set_is_ephemeral(joint.is_ephemeral());
-    // TODO(sherm1) Merge issue: do we need this?
-    // mobilizer->set_is_floating_base_mobilizer(is_floating_base_mobilizer);
+
+    // Mark floating base bodies as needed. Note the strict definition:
+    // (1) the inboard joint must have six degrees of freedom, and
+    // (2) that joint must be ephemeral (added automatically).
+    bool is_floating_base_mobilizer =
+        mobilizer->has_six_dofs() && mobilizer->is_ephemeral();
+    mobilizer->set_is_floating_base_mobilizer(is_floating_base_mobilizer);
+
     AddMobilizer(std::move(owned_mobilizer));  // ownership->tree
     DRAKE_DEMAND(mobilizer->index() == mobod.index());
     // Record the joint to mobilizer map.