aura: updates common on device scale factor caching.

BUG=665965
TEST=none

Change-Id: Icf59d9537180eeb11f572d66414e04808be23603
Reviewed-on: https://chromium-review.googlesource.com/c/1316105Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605079}

ui/aura/window_tree_host.h

@@ -352,6 +352,11 @@ class AURA_EXPORT WindowTreeHost : public ui::internal::InputMethodDelegate,
   std::unique_ptr<ui::Compositor> compositor_;
 
   // The device scale factor is snapshotted in OnHostResizedInPixels.
+  // NOTE: this value is cached rather than looked up from the Display as it is
+  // entirely possible for the Display to be updated *after* |this|. For
+  // example, display changes on Windows first result in the HWND bounds
+  // changing and are then followed by changes to the set of displays
+  //
   // TODO(ccameron): The size and location from OnHostResizedInPixels and
   // OnHostMovedInPixels should be snapshotted here as well.
   float device_scale_factor_ = 1.f;