Handle app custom icon via aura::Window property.

chrome/browser/extensions/chrome_app_icon.cc

Index: chrome/browser/extensions/chrome_app_icon.cc
diff --git a/chrome/browser/extensions/chrome_app_icon.cc b/chrome/browser/extensions/chrome_app_icon.cc
index 1b526d25dc6a81b0415773351886bc1c61b7805b..6c012f73a38bd630b35e107502a6a5c54138a112 100644
--- a/chrome/browser/extensions/chrome_app_icon.cc
+++ b/chrome/browser/extensions/chrome_app_icon.cc
@@ -90,7 +90,8 @@ const Extension* ChromeAppIcon::GetExtension() {
 void ChromeAppIcon::Reload() {
   const Extension* extension = GetExtension();
   icon_ = base::MakeUnique<IconImage>(
-      browser_context_, extension, IconsInfo::GetIcons(extension),
+      browser_context_, extension,
+      extension ? IconsInfo::GetIcons(extension) : ExtensionIconSet(),
       resource_size_in_dip_, util::GetDefaultAppIcon(), this);
   UpdateIcon();
 }