Control ES2015 'Tail call elimination' language feature via finch flag.

gin/isolate_holder.cc

Index: gin/isolate_holder.cc
diff --git a/gin/isolate_holder.cc b/gin/isolate_holder.cc
index c05cd1e42db03c95849df9c022a0315a65b2f39f..29a7f1babb92be5d8c8d8ee77c3736f1816ba22a 100644
--- a/gin/isolate_holder.cc
+++ b/gin/isolate_holder.cc
@@ -9,6 +9,7 @@
 #include <string.h>
 #include <utility>
 
+#include "base/feature_list.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
 #include "base/sys_info.h"
@@ -81,6 +82,19 @@ void IsolateHolder::Initialize(ScriptMode mode,
   CHECK(allocator);
   V8Initializer::Initialize(mode, v8_extras_mode);
   g_array_buffer_allocator = allocator;
+
+  // Initialize an instance of FeatureList. This will be a no-op if an instance
+  // was already set up by the embedder.
+  base::FeatureList::InitializeInstance();
+
+  const base::Feature kES2015TailCallsFeature {
+    "V8_ES2015_TailCalls", base::FEATURE_DISABLED_BY_DEFAULT
+  };
+  if (base::FeatureList::IsEnabled(kES2015TailCallsFeature)) {
+    static const char harmony_tailcalls[] = "--harmony-tailcalls";
+    v8::V8::SetFlagsFromString(harmony_tailcalls,
+                               sizeof(harmony_tailcalls) - 1);
+  }
 }
 
 void IsolateHolder::AddRunMicrotasksObserver() {