mus: Add debug key to print the server window hierarchy

components/mus/public/interfaces/window_manager.mojom

Index: components/mus/public/interfaces/window_manager.mojom
diff --git a/components/mus/public/interfaces/window_manager.mojom b/components/mus/public/interfaces/window_manager.mojom
index 20e0c74f761a3077ea328afda4d5588f8787a0f2..f1a4a98616b1799ad5f551a1ed4513d658275d16 100644
--- a/components/mus/public/interfaces/window_manager.mojom
+++ b/components/mus/public/interfaces/window_manager.mojom
@@ -81,6 +81,7 @@ interface WindowManagerClient {
   // target window. The id is defined by the client and can be used to more
   // easily identify the accelerator's action. If an accelerator with the same
   // id or the same matcher already exists, then the accelerator is not added.
+  // Accelerator ID 1 << 24 and above are reserved for internal use.

[msw, 2016/05/06 22:34:31]

The int32 space is 4,294,967,295, this would reserve a disproportionately large
block of that for debug accelerators (a total of 4,293,290,080 numbers are
greater than 16,777,216). Maybe use numbers 1 << 31 and above for debug, that
reserves half the available space, which still seems unnecessarily large (for
now, time will tell...), but also supports an equally unnecessarily large space
of numbers for actual accelerators.

[James Cook, 2016/05/07 00:03:10]

Done.

   AddAccelerator(uint32 id, EventMatcher matcher) => (bool success);
   RemoveAccelerator(uint32 id);