ui: Make gfx::Size::Scale() mutate the class. Add gfx::ScaleSize() similar to Rect/Point.

ash/display/multi_display_manager.cc

Index: ash/display/multi_display_manager.cc
diff --git a/ash/display/multi_display_manager.cc b/ash/display/multi_display_manager.cc
index 3f1e93d5d7bebddd6e7fc1fd4fe50e70d52f0aa0..46f1abc45199688d766ade5caf40ac4713439436 100644
--- a/ash/display/multi_display_manager.cc
+++ b/ash/display/multi_display_manager.cc
@@ -449,9 +449,10 @@ void MultiDisplayManager::ScaleDisplayImpl() {
        iter != displays_.end(); ++iter) {
     gfx::Display display = *iter;
     float factor = display.device_scale_factor() == 1.0f ? 2.0f : 1.0f;
-    display.SetScaleAndBounds(
-        factor, gfx::Rect(display.bounds_in_pixel().origin(),
-                          gfx::ToFlooredSize(display.size().Scale(factor))));
+    gfx::Point display_origin = display.bounds_in_pixel().origin();
+    gfx::Size display_size = gfx::ToFlooredSize(
+        gfx::ScaleSize(display.size(), factor));
+    display.SetScaleAndBounds(factor, gfx::Rect(display_origin, display_size));
     new_displays.push_back(display);
   }
   OnNativeDisplaysChanged(new_displays);