Add app.window.alphaEnabled() and onAlphaEnabledChanged.

chrome/browser/extensions/api/app_window/app_window_api.cc

Index: chrome/browser/extensions/api/app_window/app_window_api.cc
diff --git a/chrome/browser/extensions/api/app_window/app_window_api.cc b/chrome/browser/extensions/api/app_window/app_window_api.cc
index 199fbf4b9afcf5c70f65f9c120e15ee1d71df112..4c117501ad1bd2fcef519046d740e86469d1a833 100644
--- a/chrome/browser/extensions/api/app_window/app_window_api.cc
+++ b/chrome/browser/extensions/api/app_window/app_window_api.cc
@@ -54,6 +54,8 @@ const char kAlwaysOnTopPermission[] =
     "The \"app.window.alwaysOnTop\" permission is required.";
 const char kInvalidUrlParameter[] =
     "The URL used for window creation must be local for security reasons.";
+const char kTransparentBackgroundNeedsFrameNone[] =
+    "\"transparent_background\" can only be used with \"frame: none\".";
 }  // namespace app_window_constants
 
 const char kNoneFrameOption[] = "none";
@@ -232,12 +234,17 @@ bool AppWindowCreateFunction::RunAsync() {
     if (!GetFrameOptions(*options, &create_params))
       return false;
 
-    if (options->transparent_background.get() &&
-        (GetExtension()->permissions_data()->HasAPIPermission(
-             APIPermission::kExperimental) ||
-         CommandLine::ForCurrentProcess()->HasSwitch(
-             switches::kEnableExperimentalExtensionApis))) {
-      create_params.transparent_background = *options->transparent_background;
+    if (options->transparent_background.get()) {
+      if (create_params.frame != AppWindow::FRAME_NONE) {
+        error_ = app_window_constants::kTransparentBackgroundNeedsFrameNone;
+        return false;
+      }
+      if (GetExtension()->permissions_data()->HasAPIPermission(
+              APIPermission::kExperimental) ||
+          CommandLine::ForCurrentProcess()->HasSwitch(
+              switches::kEnableExperimentalExtensionApis)) {
+        create_params.transparent_background = *options->transparent_background;
+      }

[levin, 2014/07/15 20:40:05]

Changes in this file seem unrelated to the rest of the goals.

This change seems more about making the api more bulletproof which I applaud,
but it could this be in a separate cl (since isn't part of the core changes that
are necessary to make things work). 

imo, the smaller the cl the less risky (and the more possible to get merge
approval).

[jackhou1, 2014/07/16 01:26:29]

Seems fair, the feature is still behind a flag after all. I'll put these in
another CL.

     }
 
     if (options->hidden.get())