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_info_manager.cc

@@ -280,7 +280,9 @@ void CefBrowserInfoManager::WebContentsCreated(
 
 bool CefBrowserInfoManager::AddWebContents(content::WebContents* new_contents) {
   CEF_REQUIRE_UIT();
+#if BUILDFLAG(ENABLE_ALLOY_BOOTSTRAP)
   DCHECK(cef::IsChromeRuntimeEnabled());
+#endif
 
   // Pending popup information may be missing in cases where
   // chrome::AddWebContents is called directly from the Chrome UI (profile