⚡ Add API: SetASDefaultProtocolHandler

This PR adds an API enabling Electron to set itself as the default
protocol handler for a custom porotocl on both oS X and Windows.

For details, please see `docs/app.md`.

Closes #4857

atom/browser/api/atom_api_app.cc

@@ -370,6 +370,8 @@ mate::ObjectTemplateBuilder App::GetObjectTemplateBuilder(
                  base::Bind(&Browser::ClearRecentDocuments, browser))
       .SetMethod("setAppUserModelId",
                  base::Bind(&Browser::SetAppUserModelID, browser))
+      .SetMethod("setAsDefaultProtocolClient",
+                 base::Bind(&Browser::SetAsDefaultProtocolClient, browser))
 #if defined(OS_MACOSX)
       .SetMethod("hide", base::Bind(&Browser::Hide, browser))
       .SetMethod("show", base::Bind(&Browser::Show, browser))