Add GN arg to disable Alloy bootstrap (see #3681, see #3685)

Set enable_alloy_bootstrap=false to build with Alloy bootstrap code
removed. Extension API is documented as deprecated in comments but
not compiled out with this arg.

libcef/browser/browser_host_base.cc

@@ -731,6 +731,15 @@ void CefBrowserHostBase::NotifyMoveOrResizeStarted() {
 #endif
 }
 
+CefRefPtr<CefExtension> CefBrowserHostBase::GetExtension() {
+  NOTIMPLEMENTED();
+  return nullptr;
+}
+
+bool CefBrowserHostBase::IsBackgroundHost() {
+  return false;
+}
+
 bool CefBrowserHostBase::IsFullscreen() {
   if (!CEF_CURRENTLY_ON_UIT()) {
     DCHECK(false) << "called on invalid thread";