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* Electron version: 1.6.1
* Operating system: macOS

### Expected behavior

navigator.geolocation.getCurrentPosition runs in background

### Actual behavior

the entire UI is frozen

### How to reproduce

```js
    navigator.geolocation.getCurrentPosition((pos) => {
      console.log('POS: ' + pos);
    }, (err) => {
      console.log('Geolocation error: ', err);
    }, {enableHighAccuracy: false});
```
atom/browser/api/atom_api_web_contents.cc
.SetMethod("toggleDevTools", &WebContents::ToggleDevTools)
      .SetMethod("inspectElement", &WebContents::InspectElement)
      .SetMethod("setAudioMuted", &WebContents::SetAudioMuted)
      .SetMethod("isAudioMuted", &WebContents::IsAudioMuted)
      .SetMethod("undo", &WebContents::Undo)
      .SetMethod("redo", &WebContents::Redo)
      .SetMethod("cut", &WebContents::Cut)
      .SetMethod("copy", &WebContents::Copy)
      .SetMethod("paste", &WebContents::Paste)
      .SetMethod("pasteAndMatchStyle", &WebContents::PasteAndMatchStyle)
      .SetMethod("delete", &WebContents::Delete)
      .SetMethod("selectAll", &WebContents::SelectAll)
      .SetMethod("unselect", &WebContents::Unselect)
      .SetMethod("replace", &WebContents::Replace)
      .SetMethod("replaceMisspelling", &WebContents::ReplaceMisspelling)
      .SetMethod("findInPage", &WebContents::FindInPage)
      .SetMethod("stopFindInPage", &WebContents::StopFindInPage)
      .SetMethod("focus", &WebContents::Focus)
      .SetMethod("isFocused", &WebContents::IsFocused)
      .SetMethod("tabTraverse", &WebContents::TabTraverse)
      .SetMethod("_send", &WebContents::SendIPCMessage)
      .SetMethod("sendInputEvent", &WebContents::SendInputEvent)
      .SetMethod("beginFrameSubscription",
                 &WebContents::BeginFrameSubscription)
      .SetMethod("endFrameSubscription", &WebContents::EndFrameSubscription)
      .SetMethod("startDrag", &WebContents::StartDrag)
      .SetMethod("setSize", &WebContents::SetSize)
      .SetMethod("isGuest", &WebContents::IsGuest)
      .SetMethod("isOffscreen", &WebContents::IsOffScreen)
      .SetMethod("startPainting", &WebContents::StartPainting)