[iwyu] Include handles.h less.

src/parsing/scanner-character-streams.h

 // Copyright 2011 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.
 
 #ifndef V8_PARSING_SCANNER_CHARACTER_STREAMS_H_
 #define V8_PARSING_SCANNER_CHARACTER_STREAMS_H_
 
 #include "include/v8.h"  // for v8::ScriptCompiler
-#include "src/handles.h"
 
 namespace v8 {
 namespace internal {
 
+template <typename T>
+class Handle;
 class Utf16CharacterStream;
 class RuntimeCallStats;
+class String;
 
 class ScannerStream {
  public:
   static Utf16CharacterStream* For(Handle<String> data);
   static Utf16CharacterStream* For(Handle<String> data, int start_pos,
                                    int end_pos);
   static Utf16CharacterStream* For(
       ScriptCompiler::ExternalSourceStream* source_stream,
       ScriptCompiler::StreamedSource::Encoding encoding,
       RuntimeCallStats* stats);
 
   // For testing:
   static std::unique_ptr<Utf16CharacterStream> ForTesting(const char* data);
   static std::unique_ptr<Utf16CharacterStream> ForTesting(const char* data,
                                                           size_t length);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_SCANNER_CHARACTER_STREAMS_H_