[Gin] Add V8IgnitionEager feature.

gin/gin_features.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "gin/public/gin_features.h"
 
 namespace features {
 
 // Enables or disables the experimental V8 Ignition interpreter.
 const base::Feature kV8Ignition {
   "V8Ignition", base::FEATURE_DISABLED_BY_DEFAULT
 };
 
-// Enables or disables lazy compilation for the V8 Ignition interpreter.
+// Enables lazy compilation for the V8 Ignition interpreter. Only
+// one of V8IgnitionLazy or V8IgnitionEager should be enabled.
 const base::Feature kV8IgnitionLazy {
   "V8IgnitionLazy", base::FEATURE_DISABLED_BY_DEFAULT
 };
 
+// Enables eager compilation for the V8 Ignition interpreter. Only
+// one of V8IgnitionLazy or V8IgnitionEager should be enabled.
+const base::Feature kV8IgnitionEager {
+  "V8IgnitionEager", base::FEATURE_DISABLED_BY_DEFAULT
+};
+
 }  // namespace features