ws: Fix scaling in DesktopWindowTreeHostMus during Init

Initialize the cached display device scale factor before usage.
Otherwise, the scale factor is 1 and the window is misplaced/sized.

I'll land a unit test separately, to make merging easier.

Bug: 899084, 665965
Test: KSV (Ctrl-Alt-/) search context menu supports display & UI scales.
Change-Id: Ie74cb70af33110aa09022d4dbb5c6d0069b6cc7c
Reviewed-on: https://chromium-review.googlesource.com/c/1311080
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605543}

ui/aura/window_tree_host.cc

@@ -348,6 +348,11 @@ WindowTreeHost::WindowTreeHost(std::unique_ptr<Window> window)
   display::Screen::GetScreen()->AddObserver(this);
 }
 
+void WindowTreeHost::IntializeDeviceScaleFactor(float device_scale_factor) {
+  DCHECK(!compositor_->root_layer()) << "Only call this before InitHost()";
+  device_scale_factor_ = device_scale_factor;
+}
+
 void WindowTreeHost::DestroyCompositor() {
   if (compositor_) {
     compositor_->RemoveObserver(this);

ui/aura/window_tree_host.h

@@ -248,6 +248,10 @@ class AURA_EXPORT WindowTreeHost : public ui::internal::InputMethodDelegate,
 
   explicit WindowTreeHost(std::unique_ptr<Window> window = nullptr);
 
+  // Set the cached display device scale factor. This should only be called
+  // during subclass initialization, when the value is needed before InitHost().
+  void IntializeDeviceScaleFactor(float device_scale_factor);
+
   void DestroyCompositor();
   void DestroyDispatcher();
 

ui/views/mus/desktop_window_tree_host_mus.cc

@@ -421,8 +421,11 @@ void DesktopWindowTreeHostMus::Init(const Widget::InitParams& params) {
   if (params.show_state != ui::SHOW_STATE_DEFAULT)
     window()->SetProperty(aura::client::kShowStateKey, params.show_state);
 
-  if (!params.bounds.IsEmpty())
+  if (!params.bounds.IsEmpty()) {
+    // Init the scale now (before InitHost below), it is used by SetBoundsInDIP.
+    IntializeDeviceScaleFactor(GetDisplay().device_scale_factor());
     SetBoundsInDIP(params.bounds);
+  }
 
   cursor_manager_ = std::make_unique<wm::CursorManager>(
       std::make_unique<NativeCursorManagerMus>(window()));