Convert SetVisuallyDeemphasized IPC to mojo

extensions/renderer/mojo/frame_observer.mojom

Index: extensions/renderer/mojo/frame_observer.mojom
diff --git a/extensions/renderer/mojo/frame_observer.mojom b/extensions/renderer/mojo/frame_observer.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..aa8341a59fde43484234b5664f300e0fb396a348
--- /dev/null
+++ b/extensions/renderer/mojo/frame_observer.mojom
@@ -0,0 +1,9 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module extensions.mojom;
+
+interface FrameObserver {

[Devlin, 2017/03/04 02:19:41]

This isn't really a FrameObserver, per se (beyond the fact that the
renderer-side binding for it hangs off a RenderFrameObserver).  Since this
message is for app windows, maybe something like AppWindowHandler, or similar?

[catmullings, 2017/03/06 21:20:40]

Done.

+  SetVisuallyDeemphasized(bool deemphasized);

[Devlin, 2017/03/04 02:19:41]

Since this is a shared topic between browser and renderer, we should put it in
extensions/common (similarly to how the previous message was in chrome/common). 
That will solve the layering violations. :)

[catmullings, 2017/03/06 21:20:40]

Done.

+};