ws: Refine WindowTreeHost::device_scale_factor_ init and add a test

Attempt to init the display device scale factor in WindowTreeHost's ctor.
WindowTreeHost subclasses/users may use the cached scale before InitHost.
See http://crbug.com/899084 and https://crrev.com/c/1311080 (PS7)

This may incorrectly use the primary display's scale for new windows on extended displays.
Still, a best effort attempt to initialize the cached value here may be worthwhile.
It's unclear if this solves any practical defects similar to crbug.com/899084.

Bug: None
Test: Automated
Change-Id: Ia776d870d8a51c5ac381b2a023c7a2a579dca97a
Reviewed-on: https://chromium-review.googlesource.com/c/1318693Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605770}

ui/aura/mus/window_tree_client_unittest.cc

@@ -2412,6 +2412,20 @@ TEST_F(WindowTreeClientTestHighDPI, NewTopLevelWindowBounds) {
             top_level->GetHost()->GetBoundsInPixels());
 }
 
+TEST_F(WindowTreeClientTestHighDPI, HostCtorInitializesDisplayScale) {
+  // WindowTreeHost's ctor attempts to initialize the display scale factor.
+  WindowTreeHostMus window_tree_host(
+      CreateInitParamsForTopLevel(window_tree_client_impl()));
+  EXPECT_EQ(2.0f, window_tree_host.device_scale_factor());
+  // This test sets pixel bounds before calling WindowTreeHost::InitHost() to
+  // simulate circumstances similar to <http://crbug.com/899084>. Pixel bounds
+  // should still be scaled to DIP bounds for the window tree before InitHost.
+  gfx::Rect bounds(2, 4, 60, 80);
+  gfx::Rect pixel_bounds = gfx::ConvertRectToPixel(2.0f, bounds);
+  window_tree_host.SetBoundsInPixels(pixel_bounds);
+  EXPECT_EQ(bounds, window_tree()->last_set_window_bounds());
+}
+
 TEST_F(WindowTreeClientTestHighDPI, ObservedInputEventsInDip) {
   display::Screen* screen = display::Screen::GetScreen();
   const display::Display primary_display = screen->GetPrimaryDisplay();

ui/aura/window_tree_host.cc

@@ -348,6 +348,8 @@ WindowTreeHost::WindowTreeHost(std::unique_ptr<Window> window)
   if (!window_)
     window_ = new Window(nullptr);
   display::Screen::GetScreen()->AddObserver(this);
+  auto display = display::Screen::GetScreen()->GetDisplayNearestWindow(window_);
+  device_scale_factor_ = display.device_scale_factor();
 }
 
 void WindowTreeHost::IntializeDeviceScaleFactor(float device_scale_factor) {