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
#include "atom/browser/web_contents_preferences.h"
#include <algorithm>
#include <string>
#include <vector>
#include "atom/browser/native_window.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/options_switches.h"
#include "base/command_line.h"
#include "base/strings/string_number_conversions.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/common/content_switches.h"
#include "content/public/common/web_preferences.h"
#include "native_mate/dictionary.h"
#include "net/base/filename_util.h"
#if defined(OS_WIN)
#include "ui/gfx/switches.h"
#endif
DEFINE_WEB_CONTENTS_USER_DATA_KEY(atom::WebContentsPreferences);
namespace atom {
std::vector<WebContentsPreferences*> WebContentsPreferences::instances_;
WebContentsPreferences::WebContentsPreferences(