Remove several grab-bag includes from the v8.h header.

src/extensions/externalize-string-extension.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/extensions/externalize-string-extension.h"
 
+#include "src/api.h"
+#include "src/handles.h"
+#include "src/isolate.h"
+
 namespace v8 {
 namespace internal {
 
 template <typename Char, typename Base>
 class SimpleStringResource : public Base {
  public:
   // Takes ownership of |data|.
   SimpleStringResource(Char* data, size_t length)
       : data_(data),
         length_(length) {}
@@ skipping 112 matching lines @@
             NewStringType::kNormal).ToLocalChecked());
     return;
   }
   bool is_one_byte =
       Utils::OpenHandle(*args[0].As<v8::String>())->IsOneByteRepresentation();
   args.GetReturnValue().Set(is_one_byte);
 }
 
 }  // namespace internal
 }  // namespace v8