remove unguarded getRootNode call (#26152)

I forgot to guard the `getRootNode` call in #26106 and it fails in IE8
and old jsdom. I consolidated the implementation a bit and removed the
unguarded call

Author: gnoff

packages/react-dom-bindings/src/client/ReactDOMFloatClient.js

@@ -88,7 +88,7 @@ let lastCurrentDocument: ?Document = null;
 
 letpreviousDispatcher=null;
 exportfunctionprepareToRenderResources(rootContainer: Container){
-constrootNode=getRootNode(rootContainer);
+constrootNode=getHoistableRoot(rootContainer);
 lastCurrentDocument=getDocumentFromRoot(rootNode);
 
 previousDispatcher=Dispatcher.current;
@@ -111,7 +111,7 @@ export type HoistableRoot = Document | ShadowRoot;
 constpreloadPropsMap: Map<string,PreloadProps>=newMap();
 
 // getRootNode is missing from IE and old jsdom versions
-functiongetRootNode(container: Container): HoistableRoot{
+exportfunctiongetHoistableRoot(container: Container): HoistableRoot{
 // $FlowFixMe[method-unbinding]
 returntypeofcontainer.getRootNode==='function'
  ? /* $FlowFixMe[incompatible-return] Flow types this as returning a `Node`,
@@ -122,7 +122,7 @@ function getRootNode(container: Container): HoistableRoot {
 
 functiongetCurrentResourceRoot(): null|HoistableRoot{
 constcurrentContainer=getCurrentRootHostContainer();
-returncurrentContainer ? getRootNode(currentContainer) : null;
+returncurrentContainer ? getHoistableRoot(currentContainer) : null;
 }
 
 // Preloads are somewhat special. Even if we don't have the Document
@@ -148,11 +148,6 @@ function getDocumentFromRoot(root: HoistableRoot): Document {
 returnroot.ownerDocument||root;
 }
 
-exportfunctiongetHoistableRoot(container: Container): HoistableRoot{
-// Flow thinks getRootNode returns Node but we know it is actualy either a Document or ShadowRoot
-return((container.getRootNode(): any): Document|ShadowRoot);
-}
-
 // --------------------------------------
 // ReactDOM.Preload
 // --------------------------------------