Embedding arbitrary web content is problematic when it comes to keyboard shortcuts because:
- Web content can steal app shortcuts (see e.g. brave/browser-laptop#4407)
- Blocked web content (e.g. a focused `<webview>` performing expensive computation) will also prevent app shortcuts from firing immediately

To fix this, we need a way to _always_ handle certain shortcuts in the main process. I think the most flexible solution would be something like this:

``` js
app.addProtectedShortcut('CmdOrCtrl+Tab', (event) => {
  event.preventDefault();
});
```

We could then implement:

``` cpp
bool CommonWebContentsDelegate::PreHandleKeyboardEvent(
    content::WebContents* source,
    const content::NativeWebKeyboardEvent& event,
    bool* is_keyboard_shortcut) {
  return /* event is protected shortcut and the handler called preventDefault */;
}
```

@zcbenz Thoughts? I'm happy to work on this if we come up with a reasonable approach.

atom/browser/api/atom_api_web_contents.cc
if (type_ == REMOTE)
    return;
  if (!enable_devtools_)
    return;
  if (!managed_web_contents()->GetDevToolsWebContents())
    OpenDevTools(nullptr);
  scoped_refptr<content::DevToolsAgentHost> agent(
    content::DevToolsAgentHost::GetOrCreateFor(web_contents()));
  agent->InspectElement(x, y);
}
void WebContents::InspectServiceWorker() {
  if (type_ == REMOTE)
    return;
  if (!enable_devtools_)
    return;
  for (const auto& agent_host : content::DevToolsAgentHost::GetOrCreateAll()) {
    if (agent_host->GetType() ==
        content::DevToolsAgentHost::TYPE_SERVICE_WORKER) {
      OpenDevTools(nullptr);
      managed_web_contents()->AttachTo(agent_host);
      break;
    }
  }
}
void WebContents::HasServiceWorker(