Fix a crasher with "select file" dialogs.

app/win/window_impl.cc

Index: app/win/window_impl.cc
diff --git a/app/win/window_impl.cc b/app/win/window_impl.cc
index 9668f82ea88bd3157e29fc9b69a37129a5a274ee..54884344f42a75903ef5fa7492324e258411f8c6 100644
--- a/app/win/window_impl.cc
+++ b/app/win/window_impl.cc
@@ -152,6 +152,16 @@ HICON WindowImpl::GetDefaultWindowIcon() const {
   return NULL;
 }
 
+// static
+bool WindowImpl::IsWindowImpl(HWND hwnd) {
+  wchar_t tmp[128];
+  if (!::GetClassName(hwnd, tmp, 128))
+    return false;
+
+  std::wstring class_name(tmp);
+  return class_name.find(kBaseClassName) == 0;
+}
+
 LRESULT WindowImpl::OnWndProc(UINT message, WPARAM w_param, LPARAM l_param) {
   LRESULT result = 0;