[fix] onLayout timing without ResizeObserver

onLayout is called after the component is mounted to the DOM. This makes
both the fallback and ResizeObserver code path behave the same as React
Native.

Fix #911
Fix #941
Close #939

packages/react-native-web/src/modules/applyLayout/index.js

@@ -55,9 +55,10 @@ const observe = instance => {
     node._onLayoutId = id;
     resizeObserver.observe(node);
   } else {
-    const id = guid();
     instance._onLayoutId = id;
-    instance._handleLayout();
+    setTimeout(() => {
+      instance._handleLayout();
+    }, 0);
   }
 };
 
@@ -105,8 +106,6 @@ const applyLayout = Component => {
         observe(this);
       } else if (!this.props.onLayout && prevProps.onLayout) {
         unobserve(this);
-      } else if (!resizeObserver) {
-        this._handleLayout();
       }
     }
   );
@@ -123,10 +122,8 @@ const applyLayout = Component => {
     const layout = this._layoutState;
     const { onLayout } = this.props;
 
-    if (onLayout) {
+    if (this._isMounted && onLayout) {
       this.measure((x, y, width, height) => {
-        if (!this._isMounted) return;
-
         if (
           layout.x !== x ||
           layout.y !== y ||