refactor: turn OnOffscreen message into a command-line flag (#17687)

This turns the AtomViewMsg_Offscreen message, which only called the global setter blink::WebView::SetUseExternalPopupMenus(false) to get Chrome to render popup menus in the renderer instead of externally on macOS, into a command-line renderer flag --offscreen which does the same thing, except at render thread startup time, which is where Chromium sets the flag: https://chromium.googlesource.com/chromium/src/+/refs/tags/75.0.3755.3/content/renderer/render_thread_impl.cc#728.

This was the last usage of RenderViewObserver in our codebase, so this PR also removes that class.

atom/browser/osr/osr_web_contents_view.cc

@@ -151,10 +151,6 @@ void OffScreenWebContentsView::RenderViewCreated(
     content::RenderViewHost* host) {
   if (GetView())
     GetView()->InstallTransparency();
-
-#if defined(OS_MACOSX)
-  host->Send(new AtomViewMsg_Offscreen(host->GetRoutingID()));
-#endif
 }
 
 void OffScreenWebContentsView::RenderViewReady() {}