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.h
#ifndef ATOM_BROWSER_API_ATOM_API_WEB_CONTENTS_H_
#define ATOM_BROWSER_API_ATOM_API_WEB_CONTENTS_H_
#include <string>
#include <vector>
#include "atom/browser/api/frame_subscriber.h"
#include "atom/browser/api/save_page_handler.h"
#include "atom/browser/api/trackable_object.h"
#include "atom/browser/common_web_contents_delegate.h"
#include "content/common/cursors/webcursor.h"
#include "content/public/browser/web_contents_observer.h"
#include "content/public/common/favicon_url.h"
#include "native_mate/handle.h"
#include "ui/gfx/image/image.h"
namespace blink {
struct WebDeviceEmulationParams;
}
namespace brightray {
class InspectableWebContents;
}
namespace content {
class ResourceRequestBodyImpl;
}
namespace mate {
class Arguments;
class Dictionary;