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..36dc2c4e82bd03e5416aa34b4f0a909004962b46 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 /* max_threads */,
+                                        "MashBlocking",
+                                        base::TaskPriority::USER_VISIBLE)) {
   window_tree_client()->AddObserver(this);
   WmShell::Set(this);
 
@@ -159,6 +163,13 @@ WmShellMus::~WmShellMus() {
   if (window_tree_client())
     window_tree_client()->RemoveObserver(this);
   WmShell::Set(nullptr);
+
+  // Like BrowserThreadImpl, the goal is to make it impossible for ash to
+  // 'infinite loop' during shutdown, but to reasonably expect that all
+  // BLOCKING_SHUTDOWN tasks queued during shutdown get run. There's nothing
+  // particularly scientific about the number chosen.
+  const int kMaxNewShutdownBlockingTasks = 1000;
+  blocking_pool_->Shutdown(kMaxNewShutdownBlockingTasks);
 }
 
 // static
@@ -406,6 +417,10 @@ bool WmShellMus::IsTouchDown() {
   return false;
 }
 
+base::SequencedWorkerPool* WmShellMus::GetBlockingPool() {
+  return blocking_pool_.get();
+}
+
 #if defined(OS_CHROMEOS)
 void WmShellMus::ToggleIgnoreExternalKeyboard() {
   NOTIMPLEMENTED();