ui: Fix add/remove observer for WindowTreeHost

TBR=hubbe

Bug: 746363
Change-Id: I4aa238b281a7bd3cfd85b144109fd01a2de1fac8
Reviewed-on: https://chromium-review.googlesource.com/577721
Commit-Queue: ccameron chromium <ccameron@chromium.org>
Reviewed-by: ccameron chromium <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488051}

ui/aura/window_tree_host.cc

@@ -41,6 +41,8 @@ const char kWindowTreeHostForAcceleratedWidget[] =
 // WindowTreeHost, public:
 
 WindowTreeHost::~WindowTreeHost() {
+  if (display::Screen::GetScreen())
+    display::Screen::GetScreen()->RemoveObserver(this);
   DCHECK(!compositor_) << "compositor must be destroyed before root window";
   if (owned_input_method_) {
     delete input_method_;
@@ -224,7 +226,6 @@ void WindowTreeHost::Hide() {
 // WindowTreeHost, protected:
 
 WindowTreeHost::WindowTreeHost() : WindowTreeHost(nullptr) {
-  display::Screen::GetScreen()->AddObserver(this);
 }
 
 WindowTreeHost::WindowTreeHost(std::unique_ptr<WindowPort> window_port)
@@ -232,7 +233,7 @@ WindowTreeHost::WindowTreeHost(std::unique_ptr<WindowPort> window_port)
       last_cursor_(ui::CursorType::kNull),
       input_method_(nullptr),
       owned_input_method_(false) {
-  display::Screen::GetScreen()->RemoveObserver(this);
+  display::Screen::GetScreen()->AddObserver(this);
 }
 
 void WindowTreeHost::DestroyCompositor() {
@@ -350,13 +351,12 @@ void WindowTreeHost::OnDisplayRemoved(const display::Display& old_display) {}
 
 void WindowTreeHost::OnDisplayMetricsChanged(const display::Display& display,
                                              uint32_t metrics) {
-  display::Screen* screen = display::Screen::GetScreen();
-  if (display.id() != screen->GetDisplayNearestView(window()).id())
-    return;
-
   if (metrics & DisplayObserver::DISPLAY_METRIC_COLOR_SPACE) {
-    if (compositor_)
+    display::Screen* screen = display::Screen::GetScreen();
+    if (compositor_ &&
+        display.id() != screen->GetDisplayNearestView(window()).id()) {
       compositor_->SetDisplayColorSpace(display.color_space());
+    }
   }
 }