[Gin] Add V8IgnitionEager feature.

Make eager or lazy compilation in ignition explicit by adding a V8IgnitionEager
feature. This is to allow us to swap the default in V8 from eager to lazy
without changing the feature trail experimental groups.

BUG=v8:4868
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/1989883002
Cr-Commit-Position: refs/heads/master@{#395059}

gin/gin_features.cc

@@ -11,9 +11,16 @@ 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

gin/public/gin_features.h

@@ -15,6 +15,7 @@ namespace features {
 
 GIN_EXPORT extern const base::Feature kV8Ignition;
 GIN_EXPORT extern const base::Feature kV8IgnitionLazy;
+GIN_EXPORT extern const base::Feature kV8IgnitionEager;
 
 }  // namespace features
 

gin/v8_initializer.cc

@@ -440,13 +440,21 @@ void V8Initializer::Initialize(IsolateHolder::ScriptMode mode,
   if (base::FeatureList::IsEnabled(features::kV8Ignition)) {
     std::string flag("--ignition");
     v8::V8::SetFlagsFromString(flag.c_str(), static_cast<int>(flag.size()));
-  }
 
-  if (base::FeatureList::IsEnabled(features::kV8IgnitionLazy)) {
-    std::string flag("--no-ignition-eager");
-    v8::V8::SetFlagsFromString(flag.c_str(), static_cast<int>(flag.size()));
+    if (base::FeatureList::IsEnabled(features::kV8IgnitionEager)) {
+      std::string eager_flag("--ignition-eager");
+      v8::V8::SetFlagsFromString(
+          eager_flag.c_str(), static_cast<int>(eager_flag.size()));
+    }
+
+    if (base::FeatureList::IsEnabled(features::kV8IgnitionLazy)) {
+      std::string lazy_flag("--no-ignition-eager");
+      v8::V8::SetFlagsFromString(
+          lazy_flag.c_str(), static_cast<int>(lazy_flag.size()));
+    }
   }
 
+
 #if defined(V8_USE_EXTERNAL_STARTUP_DATA)
   v8::StartupData natives;
   natives.data = reinterpret_cast<const char*>(g_mapped_natives->data());