feedback: Make window background transparent

Bug: 997480
Change-Id: I6738a19078beb447e5e67e307ec700fc77665502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1973358
Auto-Submit: Miriam Zimmerman <mutexlox@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728757}

chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.cc

@@ -89,6 +89,12 @@ void ChromeNativeAppWindowViewsAuraAsh::InitializeWindow(
   // Fullscreen doesn't always imply immersive mode (see
   // ShouldEnableImmersive()).
   window->SetProperty(ash::kImmersiveImpliedByFullscreen, false);
+  // TODO(https://crbug.com/997480): Determine if all non-resizable windows
+  // should have this behavior, or just the feedback app.
+  if (app_window->extension_id() == extension_misc::kFeedbackExtensionId) {
+    window->SetProperty(ash::kBackdropWindowMode,
+                        ash::BackdropWindowMode::kAutoSemiOpaque);
+  }
   observed_window_.Add(window);
 }