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/features/runtime_checks.h

@@ -6,13 +6,18 @@
 #define CEF_LIBCEF_FEATURES_RUNTIME_CHECKS_H_
 #pragma once
 
-#include "base/logging.h"
 #include "cef/libcef/features/runtime.h"
 
+#if BUILDFLAG(ENABLE_ALLOY_BOOTSTRAP)
+
+#include "base/logging.h"
+
 #define REQUIRE_ALLOY_RUNTIME() \
   CHECK(cef::IsAlloyRuntimeEnabled()) << "Alloy runtime is required"
 
 #define REQUIRE_CHROME_RUNTIME() \
   CHECK(cef::IsChromeRuntimeEnabled()) << "Chrome runtime is required"
 
+#endif  // BUILDFLAG(ENABLE_ALLOY_BOOTSTRAP)
+
 #endif  // CEF_LIBCEF_FEATURES_RUNTIME_CHECKS_H_