Flip --enable-apps-show-on-first-paint flag.

chrome/renderer/resources/extensions/app_window_custom_bindings.js

Index: chrome/renderer/resources/extensions/app_window_custom_bindings.js
diff --git a/chrome/renderer/resources/extensions/app_window_custom_bindings.js b/chrome/renderer/resources/extensions/app_window_custom_bindings.js
index 87ec1b3edddb6be35a26f438eef31bca8f6e552e..770497394415806c91b225526032e976ac9d6627 100644
--- a/chrome/renderer/resources/extensions/app_window_custom_bindings.js
+++ b/chrome/renderer/resources/extensions/app_window_custom_bindings.js
@@ -213,6 +213,7 @@ appWindow.registerCustomHook(function(bindingsAPI) {
     AppWindow.prototype.resizeTo = $Function.bind(window.resizeTo, window);
     AppWindow.prototype.contentWindow = window;
     AppWindow.prototype.onClosed = new Event();
+    AppWindow.prototype.onWindowFirstShown = new Event();
     AppWindow.prototype.close = function() {
       this.contentWindow.close();
     };
@@ -340,6 +341,13 @@ function updateAppWindowProperties(update) {
     dispatchEventIfExists(currentWindow, "onRestored");
 };
 
+function onAppWindowFirstShown() {
+  if (!currentAppWindow)
+    return;
+
+  dispatchEventIfExists(currentAppWindow, "onWindowFirstShown");
+}
+
 function onAppWindowClosed() {
   if (!currentAppWindow)
     return;
@@ -371,3 +379,4 @@ function updateSizeConstraints(boundsType, constraints) {
 exports.binding = appWindow.generate();
 exports.onAppWindowClosed = onAppWindowClosed;
 exports.updateAppWindowProperties = updateAppWindowProperties;
+exports.appWindowFirstShown = onAppWindowFirstShown;