[v8] Add feature flag for off-thread streaming finalization

Adds a feature flag for enabling off-thread finalization of V8 streaming
compilation tasks. This feature should reduce main-thread script
compilation time.

Bug: 1011762
Change-Id: I64bb986e07dae8d68dc61269e69096c368b1aac8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135629
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756313}

gin/gin_features.cc

@@ -14,6 +14,10 @@ const base::Feature kV8OptimizeJavascript{"V8OptimizeJavascript",
 const base::Feature kV8FlushBytecode{"V8FlushBytecode",
                                      base::FEATURE_ENABLED_BY_DEFAULT};
 
+// Enables finalizing streaming JS compilations on a background thread.
+const base::Feature kV8OffThreadFinalization{"V8OffThreadFinalization",
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Enables lazy feedback allocation in V8.
 const base::Feature kV8LazyFeedbackAllocation{"V8LazyFeedbackAllocation",
                                               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 kV8OffThreadFinalization;
 GIN_EXPORT extern const base::Feature kV8LazyFeedbackAllocation;
 GIN_EXPORT extern const base::Feature kV8ConcurrentInlining;
 GIN_EXPORT extern const base::Feature kV8PerContextMarkingWorklist;

gin/v8_initializer.cc

@@ -211,6 +211,13 @@ void V8Initializer::Initialize(IsolateHolder::ScriptMode mode) {
                                sizeof(no_flush_bytecode) - 1);
   }
 
+  if (base::FeatureList::IsEnabled(features::kV8OffThreadFinalization)) {
+    static const char finalize_streaming_on_background[] =
+        "--finalize-streaming-on-background";
+    v8::V8::SetFlagsFromString(finalize_streaming_on_background,
+                               sizeof(finalize_streaming_on_background) - 1);
+  }
+
   if (!base::FeatureList::IsEnabled(features::kV8LazyFeedbackAllocation)) {
     static const char no_lazy_feedback_allocation[] =
         "--no-lazy-feedback-allocation";