Fix tab drag and drop with hi-dpi.

ui/views/widget/desktop_aura/x11_topmost_window_finder.cc

Index: ui/views/widget/desktop_aura/x11_topmost_window_finder.cc
diff --git a/ui/views/widget/desktop_aura/x11_topmost_window_finder.cc b/ui/views/widget/desktop_aura/x11_topmost_window_finder.cc
index d59b46cf3798755f1bfc8a11995b0723e64cbc18..71330483e6037851fa47aa6b2e7317ccd569d163 100644
--- a/ui/views/widget/desktop_aura/x11_topmost_window_finder.cc
+++ b/ui/views/widget/desktop_aura/x11_topmost_window_finder.cc
@@ -8,10 +8,20 @@
 
 #include "ui/aura/client/screen_position_client.h"
 #include "ui/aura/window.h"
+#include "ui/gfx/screen.h"
 #include "ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h"
 
 namespace views {
 
+float GetDeviceScaleFactor() {
+  gfx::Display display = gfx::Screen::GetNativeScreen()->GetPrimaryDisplay();
+  return display.device_scale_factor();
+}
+
+gfx::Point DIPToPixelPoint(const gfx::Point& dip_point) {
+  return ToFlooredPoint(gfx::ScalePoint(dip_point, GetDeviceScaleFactor()));
+}
+
 X11TopmostWindowFinder::X11TopmostWindowFinder() : toplevel_(None) {
 }
 
@@ -21,7 +31,7 @@ X11TopmostWindowFinder::~X11TopmostWindowFinder() {
 aura::Window* X11TopmostWindowFinder::FindLocalProcessWindowAt(
     const gfx::Point& screen_loc,
     const std::set<aura::Window*>& ignore) {
-  screen_loc_ = screen_loc;
+  screen_loc_ = DIPToPixelPoint(screen_loc);

[sadrul, 2015/03/19 23:46:58]

Since FindWindowAt() takes screen_loc in physical-pixel space, I think it would
be better to have FindLocalProcessWindowAt() also take screen_loc in
physical-pixels. Please make sure to append |in_pixels| to the variable names to
clarify this.

[stapelberg, 2015/03/20 08:34:40]

Done.

   ignore_ = ignore;
 
   std::vector<aura::Window*> local_process_windows =