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.
