Add support for transparent background API

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..cfeb11bb2034cdd4d18b7ba182153f1a41f7fe69 100644
--- a/chrome/renderer/resources/extensions/app_window_custom_bindings.js
+++ b/chrome/renderer/resources/extensions/app_window_custom_bindings.js
@@ -240,6 +240,9 @@ appWindow.registerCustomHook(function(bindingsAPI) {
     AppWindow.prototype.isAlwaysOnTop = function() {
       return appWindowData.alwaysOnTop;
     };
+    AppWindow.prototype.alphaEnabled = function() {
+      return appWindowData.alphaEnabled;
+    };
 
     Object.defineProperty(AppWindow.prototype, 'id', {get: function() {
       return appWindowData.id;
@@ -289,6 +292,7 @@ appWindow.registerCustomHook(function(bindingsAPI) {
       minimized: params.minimized,
       maximized: params.maximized,
       alwaysOnTop: params.alwaysOnTop,
+      alphaEnabled: params.alphaEnabled,
       hasFrameColor: params.hasFrameColor,
       activeFrameColor: params.activeFrameColor,
       inactiveFrameColor: params.inactiveFrameColor
@@ -338,6 +342,9 @@ function updateAppWindowProperties(update) {
       (oldData.minimized && !update.minimized) ||
       (oldData.maximized && !update.maximized))
     dispatchEventIfExists(currentWindow, "onRestored");
+
+  if (oldData.alphaEnabled != update.alphaEnabled)
+    dispatchEventIfExists(currentWindow, "onAlphaEnabledChanged");
 };
 
 function onAppWindowClosed() {