Handle app custom icon via aura::Window property.

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 355a01da13111e63dc3ce843cd8e6818cc85c2f0..c96de9493f4e3efa67deb49de0b5c4b7337dd20a 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -2025,7 +2025,11 @@ test("browser_tests") {
       }
     }
     if (use_aura) {
-      sources += [ "../browser/ui/aura/accessibility/automation_manager_aura_browsertest.cc" ]
+      sources += [
+        "../browser/ui/aura/accessibility/automation_manager_aura_browsertest.cc",
+        "../browser/ui/test/test_app_window_icon_observer.cc",
+        "../browser/ui/test/test_app_window_icon_observer.h",
+      ]
     }
     if (use_aura || toolkit_views) {
       deps += [ "//ui/events:test_support" ]