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
.SetMethod("isOffscreen", &WebContents::IsOffScreen)
      .SetMethod("startPainting", &WebContents::StartPainting)
      .SetMethod("stopPainting", &WebContents::StopPainting)
      .SetMethod("isPainting", &WebContents::IsPainting)
      .SetMethod("setFrameRate", &WebContents::SetFrameRate)
      .SetMethod("getFrameRate", &WebContents::GetFrameRate)
      .SetMethod("invalidate", &WebContents::Invalidate)
      .SetMethod("getType", &WebContents::GetType)
      .SetMethod("getWebPreferences", &WebContents::GetWebPreferences)
      .SetMethod("getOwnerBrowserWindow", &WebContents::GetOwnerBrowserWindow)
      .SetMethod("hasServiceWorker", &WebContents::HasServiceWorker)
      .SetMethod("unregisterServiceWorker",
                 &WebContents::UnregisterServiceWorker)
      .SetMethod("inspectServiceWorker", &WebContents::InspectServiceWorker)
      .SetMethod("print", &WebContents::Print)
      .SetMethod("_printToPDF", &WebContents::PrintToPDF)
      .SetMethod("addWorkSpace", &WebContents::AddWorkSpace)
      .SetMethod("removeWorkSpace", &WebContents::RemoveWorkSpace)
      .SetMethod("showDefinitionForSelection",
                 &WebContents::ShowDefinitionForSelection)
      .SetMethod("copyImageAt", &WebContents::CopyImageAt)
      .SetMethod("capturePage", &WebContents::CapturePage)
      .SetMethod("setEmbedder", &WebContents::SetEmbedder)
      .SetProperty("id", &WebContents::ID)
      .SetProperty("session", &WebContents::Session)
      .SetProperty("hostWebContents", &WebContents::HostWebContents)
      .SetProperty("devToolsWebContents", &WebContents::DevToolsWebContents)
      .SetProperty("debugger", &WebContents::Debugger);
}
AtomBrowserContext* WebContents::GetBrowserContext() const {