Adds a class that makes it easy to subclass a HWND and filter messages sent to it.

ui/views/views.gyp

Index: ui/views/views.gyp
===================================================================
--- ui/views/views.gyp	(revision 134136)
+++ ui/views/views.gyp	(working copy)
@@ -322,6 +322,8 @@
         'widget/drop_helper.h',
         'widget/drop_target_win.cc',
         'widget/drop_target_win.h',
+        'widget/hwnd_subclass.cc',
+        'widget/hwnd_subclass.h',
         'widget/root_view.cc',
         'widget/root_view.h',
         'widget/tooltip_manager_aura.cc',
@@ -426,6 +428,11 @@
           'include_dirs': [
             '../../third_party/wtl/include',
           ],
+        }, {  # OS!="win"
+          'sources!': [
+            'widget/hwnd_subclass.cc',
+            'widget/hwnd_subclass.h',
+          ],
         }],
         ['use_x11==0', {
           'sources!': [
@@ -496,6 +503,7 @@
         'view_model_unittest.cc',
         'view_model_utils_unittest.cc',
         'view_unittest.cc',
+        'widget/hwnd_subclass_unittest.cc',
         'widget/native_widget_aura_unittest.cc',
         'widget/native_widget_test_utils.h',
         'widget/native_widget_test_utils_aura.cc',
@@ -520,6 +528,10 @@
           'include_dirs': [
             '../third_party/wtl/include',
           ],
+        }, {  # OS!="win"
+          'sources!': [
+            'widget/hwnd_subclass_unittest.cc',
+          ],
         }],
         ['use_aura==0 and OS=="win"', {
           'sources/': [