- Electron version: 1.3.6
- Operating system: Windows 7 x64
- Other: Microsoft Office 2013 (will make more sense in a bit)

`shell.openItem()` is a blocking call, it would be nice if it's async or has an async version to handle synchronous external application behavior.

Reproduce: 
Open a password-protected Microsoft Excel file, `shell.openItem()` will be blocking until the Password window is submitted or dismissed. Consequently Microsoft Word does not have this issue.

Related to #2761.

shell/browser/ui/inspectable_web_contents_impl.cc
const char kChromeUIDevToolsRemoteFrontendPath[] = "serve_file";
const char kDevToolsBoundsPref[] = "electron.devtools.bounds";
const char kDevToolsZoomPref[] = "electron.devtools.zoom";
const char kDevToolsPreferences[] = "electron.devtools.preferences";
const char kFrontendHostId[] = "id";
const char kFrontendHostMethod[] = "method";
const char kFrontendHostParams[] = "params";
const char kTitleFormat[] = "Developer Tools - %s";
const size_t kMaxMessageChunkSize = IPC::Channel::kMaximumMessageSize / 4;
InspectableWebContentsImpl::List g_web_contents_instances_;
base::Value RectToDictionary(const gfx::Rect& bounds) {
  base::Value dict(base::Value::Type::DICTIONARY);
  dict.SetKey("x", base::Value(bounds.x()));
  dict.SetKey("y", base::Value(bounds.y()));
  dict.SetKey("width", base::Value(bounds.width()));
  dict.SetKey("height", base::Value(bounds.height()));
  return dict;
}
gfx::Rect DictionaryToRect(const base::Value* dict) {
  const base::Value* found = dict->FindKey("x");
  int x = found ? found->GetInt() : 0;
  found = dict->FindKey("y");
  int y = found ? found->GetInt() : 0;
  found = dict->FindKey("width");
  int width = found ? found->GetInt() : 800;
  found = dict->FindKey("height");
  int height = found ? found->GetInt() : 600;
  return gfx::Rect(x, y, width, height);
}
bool IsPointInRect(const gfx::Point& point, const gfx::Rect& rect) {