[gin] Add a feature flag for a reverse Finch of V8's --local-heaps

Bug: 1123904
Change-Id: I6dbfa593f899c970935a825fb22ca9572879c256
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386802Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803506}

gin/gin_features.cc

@@ -42,4 +42,8 @@ const base::Feature kV8ReduceConcurrentMarkingTasks{
 const base::Feature kV8NoReclaimUnmodifiedWrappers{
     "V8NoReclaimUnmodifiedWrappers", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Enables concurrent heap access and allocation.
+const base::Feature kV8LocalHeaps{"V8LocalHeaps",
+                                  base::FEATURE_ENABLED_BY_DEFAULT};
+
 }  // namespace features

gin/gin_features.h

@@ -19,6 +19,7 @@ GIN_EXPORT extern const base::Feature kV8PerContextMarkingWorklist;
 GIN_EXPORT extern const base::Feature kV8FlushEmbeddedBlobICache;
 GIN_EXPORT extern const base::Feature kV8ReduceConcurrentMarkingTasks;
 GIN_EXPORT extern const base::Feature kV8NoReclaimUnmodifiedWrappers;
+GIN_EXPORT extern const base::Feature kV8LocalHeaps;
 
 }  // namespace features
 

gin/v8_initializer.cc

@@ -263,6 +263,13 @@ void V8Initializer::Initialize(IsolateHolder::ScriptMode mode) {
                                sizeof(no_reclaim_unmodified_wrappers) - 1);
   }
 
+  if (!base::FeatureList::IsEnabled(features::kV8LocalHeaps)) {
+    // The --local-heaps flag is enabled by default, so we need to explicitly
+    // disabled it if kV8LocalHeaps is disabled.
+    static constexpr char no_local_heaps[] = "--no-local-heaps";
+    v8::V8::SetFlagsFromString(no_local_heaps, sizeof(no_local_heaps) - 1);
+  }
+
   if (IsolateHolder::kStrictMode == mode) {
     static const char use_strict[] = "--use_strict";
     v8::V8::SetFlagsFromString(use_strict, sizeof(use_strict) - 1);