* Electron version: 1.6.3
* Operating system: Linux

### Expected behavior

When registering a `beforeunload` handler which cancels unloading, I would expect the handler to be called for every `BrowserWindow.reload()`.

### Actual behavior

On 1.6.3, the `beforeunload` handler is called for the first `reload` but not on subsequent calls. Even after removing the event handler, `reload` will not reload the window anymore.

### How to reproduce

```javascript
const cancelUnload = e => {
   console.log('cancelUnload called')
   e.returnValue = false
}

window.addEventListener('beforeunload', cancelUnload)

remote.getCurrentWindow().reload()  // [1] -> cancelUnload called
remote.getCurrentWindow().reload()  // [2] -> cancelUnload called

remote.getCurrentWindow().close()   // [3] -> cancelUnload called
remote.getCurrentWindow().close()   // [4] -> cancelUnload called

window.removeEventListener('beforeunload', cancelUnload)

remote.getCurrentWindow().reload()  // [5] -> reloads window

remote.getCurrentWindow().close()   // [6] -> closes window
```
I would expect `cancelUnload` to be printed to the console four times and the step at `[5]` to actually reload the window. This is works up to Electron 1.6.2.

On Electron 1.6.3 `cancelUnload` is not printed at step `[2]` (nor on any subsequent calls to reload). What's more, event after removing the `beforeunload` handler, `reload` does not work anymore: at step `[5]` the window does not reload (but closing the window at `[6]` works).
atom/browser/api/atom_api_web_contents.cc
bitmap_size = gfx::ScaleToCeiledSize(view_size, scale);
  host->CopyFromBackingStore(gfx::Rect(rect.origin(), view_size),
                             bitmap_size,
                             base::Bind(&OnCapturePageDone, callback),
                             kBGRA_8888_SkColorType);
}
void WebContents::OnCursorChange(const content::WebCursor& cursor) {
  content::WebCursor::CursorInfo info;
  cursor.GetCursorInfo(&info);
  if (cursor.IsCustom()) {
    Emit("cursor-changed", CursorTypeToString(info),
      gfx::Image::CreateFrom1xBitmap(info.custom_image),
      info.image_scale_factor,
      gfx::Size(info.custom_image.width(), info.custom_image.height()),
      info.hotspot);
  } else {
    Emit("cursor-changed", CursorTypeToString(info));
  }
}
void WebContents::SetSize(const SetSizeParams& params) {
  if (guest_delegate_)
    guest_delegate_->SetSize(params);
}
bool WebContents::IsGuest() const {
  return type_ == WEB_VIEW;
}
bool WebContents::IsOffScreen() const {
  return type_ == OFF_SCREEN;
}
void WebContents::OnPaint(const gfx::Rect& dirty_rect, const SkBitmap& bitmap) {