[gin] Add a feature flag to control lazy feedback allocation in V8

We would like to run a finch trial on this feature.

Bug: v8:8394
Change-Id: Iecac081955574c4add95627e10c3dbd5eda547c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642640Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666243}

gin/gin_features.cc

@@ -14,6 +14,10 @@ const base::Feature kV8OptimizeJavascript{"V8OptimizeJavascript",
 const base::Feature kV8FlushBytecode{"V8FlushBytecode",
                                      base::FEATURE_ENABLED_BY_DEFAULT};
 
+// Enables lazy feedback allocation in V8.
+const base::Feature kV8LazyFeedbackAllocation{"V8LazyFeedbackAllocation",
+                                              base::FEATURE_ENABLED_BY_DEFAULT};
+
 // Enables memory reducer for small heaps in V8.
 const base::Feature kV8MemoryReducerForSmallHeaps{
     "V8MemoryReducerForSmallHeaps", base::FEATURE_ENABLED_BY_DEFAULT};

gin/gin_features.h

@@ -12,6 +12,7 @@ namespace features {
 
 GIN_EXPORT extern const base::Feature kV8OptimizeJavascript;
 GIN_EXPORT extern const base::Feature kV8FlushBytecode;
+GIN_EXPORT extern const base::Feature kV8LazyFeedbackAllocation;
 GIN_EXPORT extern const base::Feature kV8MemoryReducerForSmallHeaps;
 GIN_EXPORT extern const base::Feature kV8HugeMaxOldGenerationSize;
 

gin/v8_initializer.cc

@@ -225,6 +225,13 @@ void V8Initializer::Initialize(IsolateHolder::ScriptMode mode) {
                                sizeof(no_flush_bytecode) - 1);
   }
 
+  if (!base::FeatureList::IsEnabled(features::kV8LazyFeedbackAllocation)) {
+    static const char no_lazy_feedback_allocation[] =
+        "--no-lazy-feedback-allocation";
+    v8::V8::SetFlagsFromString(no_lazy_feedback_allocation,
+                               sizeof(no_lazy_feedback_allocation) - 1);
+  }
+
   if (!base::FeatureList::IsEnabled(features::kV8MemoryReducerForSmallHeaps)) {
     static const char no_memory_reducer[] =
         "--no-memory-reducer-for-small-heaps";