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
void InspectElement(int x, int y);
  void InspectServiceWorker();
  void HasServiceWorker(const base::Callback<void(bool)>&);
  void UnregisterServiceWorker(const base::Callback<void(bool)>&);
  void SetAudioMuted(bool muted);
  bool IsAudioMuted();
  void Print(mate::Arguments* args);
  void SetEmbedder(const WebContents* embedder);
  
  void PrintToPDF(const base::DictionaryValue& setting,
                  const PrintToPDFCallback& callback);
  
  void AddWorkSpace(mate::Arguments* args, const base::FilePath& path);
  void RemoveWorkSpace(mate::Arguments* args, const base::FilePath& path);
  
  void Undo();
  void Redo();
  void Cut();
  void Copy();
  void Paste();
  void PasteAndMatchStyle();
  void Delete();
  void SelectAll();
  void Unselect();
  void Replace(const base::string16& word);
  void ReplaceMisspelling(const base::string16& word);
  uint32_t FindInPage(mate::Arguments* args);
  void StopFindInPage(content::StopFindAction action);
  void ShowDefinitionForSelection();
  void CopyImageAt(int x, int y);