[gin] Add a feature for unsafe fast JS calls

The feature is disabled by default, as is the current state of the flag
in V8. This CL is intended to allow us to start a Finch experiment for
enabling the feature over time.

Bug: chromium:1052746
Change-Id: I8f7f29fe7661bfeb911c63e0a892905efb3edb80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632682Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844752}

gin/gin_features.cc

@@ -54,4 +54,8 @@ const base::Feature kV8TurboDirectHeapAccess{"V8TurboDirectHeapAccess",
 const base::Feature kV8ExperimentalRegexpEngine{
     "V8ExperimentalRegexpEngine", base::FEATURE_ENABLED_BY_DEFAULT};
 
+// Enables fast API calls in TurboFan.
+const base::Feature kV8TurboFastApiCalls{"V8TurboFastApiCalls",
+                                         base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features

gin/gin_features.h

@@ -22,6 +22,7 @@ GIN_EXPORT extern const base::Feature kV8NoReclaimUnmodifiedWrappers;
 GIN_EXPORT extern const base::Feature kV8LocalHeaps;
 GIN_EXPORT extern const base::Feature kV8TurboDirectHeapAccess;
 GIN_EXPORT extern const base::Feature kV8ExperimentalRegexpEngine;
+GIN_EXPORT extern const base::Feature kV8TurboFastApiCalls;
 
 }  // namespace features
 

gin/v8_initializer.cc

@@ -298,6 +298,12 @@ void V8Initializer::Initialize(IsolateHolder::ScriptMode mode) {
                                sizeof(no_experimental_regexp_engine) - 1);
   }
 
+  if (base::FeatureList::IsEnabled(features::kV8TurboFastApiCalls)) {
+    static const char turbo_fast_api_calls[] = "--turbo-fast-api-calls";
+    v8::V8::SetFlagsFromString(turbo_fast_api_calls,
+                               sizeof(turbo_fast_api_calls) - 1);
+  }
+
   if (IsolateHolder::kStrictMode == mode) {
     static const char use_strict[] = "--use_strict";
     v8::V8::SetFlagsFromString(use_strict, sizeof(use_strict) - 1);