mash: Migrate wallpaper controllers to ash/common.

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index 78959a48aad559e9114e08841a787df655b60d2d..47470e1ae676168096fab17c03d59a641554dcf9 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -113,7 +113,11 @@ WmShellMus::WmShellMus(
     : WmShell(std::move(shell_delegate)),
       window_manager_(window_manager),
       pointer_watcher_event_router_(pointer_watcher_event_router),
-      session_state_delegate_(new SessionStateDelegateStub) {
+      session_state_delegate_(new SessionStateDelegateStub),
+      blocking_pool_(
+          new base::SequencedWorkerPool(3,

[James Cook, 2016/09/07 21:27:05]

Do you need to call blocking_pool_->Shutdown()?

Also, I'm not too familiar with SequencedWorkerPool. You might want to have
someone else look at this part. I seem to recall sky@ had an issue with shutdown
leaks doing something similar to this (even with Shutdown). See
https://codereview.chromium.org/2296973002

Also nit: It would be nice to comment what "3" is.

[msw, 2016/09/07 22:28:24]

Done; that fixes mash_unittests. I'll also ask Scott to check this.

[sky, 2016/09/08 04:15:22]

I believe you want to make it so you can inject a sequencedworkerpool for tests.
See base::SequencedWorkerPoolOwner for details on all the reasons you may run
into problems if you don't use SequencedWorkerPoolOwner in tests.

[msw, 2016/09/08 19:52:15]

Done; I moved Shell::blocking_pool_ to WmShell, made WindowManagerApplication
create/own the pool for mash (except in tests, afaict), and made WmTestHelper
own a SequencedWorkerPoolOwner for tests. This required some
WindowManager[Application] initialization plumbing. Please let me know if you
see a better approach.

+                                        "MashBlocking",
+                                        base::TaskPriority::USER_VISIBLE)) {
   window_tree_client()->AddObserver(this);
   WmShell::Set(this);
 
@@ -406,6 +410,10 @@ bool WmShellMus::IsTouchDown() {
   return false;
 }
 
+base::SequencedWorkerPool* WmShellMus::GetBlockingPool() {
+  return blocking_pool_.get();
+}
+
 #if defined(OS_CHROMEOS)
 void WmShellMus::ToggleIgnoreExternalKeyboard() {
   NOTIMPLEMENTED();