MacViews: Implement Tab Dragging

chrome/browser/ui/views/tabs/window_finder_mac.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/views/tabs/window_finder.h"
 
+#include <AppKit/AppKit.h>
+
+#include "ui/gfx/geometry/point.h"
+
 gfx::NativeWindow GetLocalProcessWindowAtPoint(
     const gfx::Point& screen_point,
     const std::set<gfx::NativeWindow>& ignore) {
-  NOTIMPLEMENTED();
-  return NULL;
+  NSPoint ns_point = NSMakePoint(
+      screen_point.x(),
+      NSMaxY([[[NSScreen screens] objectAtIndex:0] frame]) - screen_point.y());
+
+  // Note [NSApp orderedWindows] doesn't include NSPanels.
+  for (NSWindow* window : [NSApp orderedWindows]) {
+    if (ignore.count(window))
+      continue;
+
+    if (![window isOnActiveSpace])
+      continue;
+
+    if (NSPointInRect(ns_point, [window frame])) {
+      return window;
+    }
+  }
+
+  return nil;
 }