Refactor IsolateHolder to be able to always create the isolate

gin/public/isolate_holder.h

Index: gin/public/isolate_holder.h
diff --git a/gin/public/isolate_holder.h b/gin/public/isolate_holder.h
index ee696f6550bb2220c6f169d07653699e3cb84896..af01782761ecf5ab3e315e214e4d10d835457300 100644
--- a/gin/public/isolate_holder.h
+++ b/gin/public/isolate_holder.h
@@ -32,11 +32,17 @@ class GIN_EXPORT IsolateHolder {
     kStrictMode
   };
 
-  explicit IsolateHolder(ScriptMode mode);
+  IsolateHolder();
+  // Deprecated.
   IsolateHolder(v8::Isolate* isolate, v8::ArrayBuffer::Allocator* allocator);
 
   ~IsolateHolder();
 
+  // Should be invoked once before creating IsolateHolder instances to
+  // initialize V8 and Gin.
+  static void Initialize(ScriptMode mode,
+                         v8::ArrayBuffer::Allocator* allocator);
+
   v8::Isolate* isolate() { return isolate_; }
 
  private: