Changes how window is updated when display changes in mash

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index a2cfe7bc506b5b793da965bf13fa1a4eed222bc6..1295ca50553fba17fb225fd0a578d5c3206b9be9 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -485,7 +485,26 @@ void WindowManager::OnWmDisplayRemoved(
 }
 
 void WindowManager::OnWmDisplayModified(const display::Display& display) {
-  screen_->display_list().UpdateDisplay(display);
+  // Ash relies on the Display being updated, then the WindowTreeHost's window,
+  // and finally DisplayObservers.
+  std::unique_ptr<display::DisplayListObserverLock> display_lock =
+      screen_->display_list().SuspendObserverUpdates();
+  const bool is_primary =
+      screen_->display_list().FindDisplayById(display.id()) ==
+      screen_->display_list().GetPrimaryDisplayIterator();
+  uint32_t display_changed_values = screen_->display_list().UpdateDisplay(
+      display, is_primary ? display::DisplayList::Type::PRIMARY
+                          : display::DisplayList::Type::NOT_PRIMARY);
+  RootWindowController* root_window_controller =
+      WmShellMus::Get()->GetRootWindowControllerWithDisplayId(display.id());
+  DCHECK(root_window_controller);
+  root_window_controller->GetRootWindow()->GetHost()->SetBoundsInPixels(
+      display.bounds());
+  display_lock.reset();
+  for (display::DisplayObserver& observer :
+       *(screen_->display_list().observers())) {
+    observer.OnDisplayMetricsChanged(display, display_changed_values);
+  }

[James Cook, 2017/03/30 15:34:34]

optional: line wrapping in this function might be better with "using
display::DisplayList" and/or putting screen_->display_list() in a local.

[sky, 2017/03/30 19:04:24]

I went with making a local for screen_->display_list().

 }
 
 void WindowManager::OnWmPerformMoveLoop(