Clarify that the transitionLanes used by Transition Tracing is a LaneMap

Author: sebmarkbage

packages/react-reconciler/src/ReactFiberRoot.js

@@ -112,10 +112,7 @@ function FiberRootNode(
   this.incompleteTransitions = new Map();
   if (enableTransitionTracing) {
     this.transitionCallbacks = null;
-    const transitionLanesMap = (this.transitionLanes = []);
-    for (let i = 0; i < TotalLanes; i++) {
-      transitionLanesMap.push(null);
-    }
+    this.transitionLanes = createLaneMap(null);
   }
 
   if (enableProfilerTimer && enableProfilerCommitHooks) {

packages/react-reconciler/src/ReactInternalTypes.js

@@ -357,7 +357,7 @@ export type TransitionTracingCallbacks = {
 // The following fields are only used in transition tracing in Profile builds
 type TransitionTracingOnlyFiberRootProperties = {
   transitionCallbacks: null | TransitionTracingCallbacks,
-  transitionLanes: Array<Set<Transition> | null>,
+  transitionLanes: LaneMap<Set<Transition> | null>,
   // Transitions on the root can be represented as a bunch of tracing markers.
   // Each entangled group of transitions can be treated as a tracing marker.
   // It will have a set of pending suspense boundaries. These transitions