It doesn't look very good for "native" apps to overscroll when scrolling. Would be nice to have a BrowserWindow option to disable overscroll

![untitled12](https://cloud.githubusercontent.com/assets/170270/15033357/fc36e32e-1293-11e6-8cdc-6c17b0f504dd.gif)
## 
- Electron version: 0.37.8
- Operating system: OS X 10.11.4

atom/browser/web_contents_preferences.cc
content::WebContents* web_contents, content::WebPreferences* prefs) {
  WebContentsPreferences* self = FromWebContents(web_contents);
  if (!self)
    return;
  bool b;
  if (self->web_preferences_.GetBoolean("javascript", &b))
    prefs->javascript_enabled = b;
  if (self->web_preferences_.GetBoolean("images", &b))
    prefs->images_enabled = b;
  if (self->web_preferences_.GetBoolean("textAreasAreResizable", &b))
    prefs->text_areas_are_resizable = b;
  if (self->web_preferences_.GetBoolean("webgl", &b))
    prefs->experimental_webgl_enabled = b;
  if (self->web_preferences_.GetBoolean("webaudio", &b))
    prefs->webaudio_enabled = b;
  if (self->web_preferences_.GetBoolean("webSecurity", &b)) {
    prefs->web_security_enabled = b;
    prefs->allow_displaying_insecure_content = !b;
    prefs->allow_running_insecure_content = !b;