Add a few missing overrides found by a new clang warning.

src/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_SCANNER_CHARACTER_STREAMS_H_
 #define V8_SCANNER_CHARACTER_STREAMS_H_
 
 #include "src/scanner.h"
 
 namespace v8 {
@@ skipping 82 matching lines @@
 
   virtual unsigned BufferSeekForward(unsigned delta) OVERRIDE {
     // We never need to seek forward when streaming scripts. We only seek
     // forward when we want to parse a function whose location we already know,
     // and when streaming, we don't know the locations of anything we haven't
     // seen yet.
     UNREACHABLE();
     return 0;
   }
 
-  virtual unsigned FillBuffer(unsigned position);
+  virtual unsigned FillBuffer(unsigned position) OVERRIDE;
 
  private:
   void HandleUtf8SplitCharacters(unsigned* data_in_buffer);
 
   ScriptCompiler::ExternalSourceStream* source_stream_;
   v8::ScriptCompiler::StreamedSource::Encoding encoding_;
   const uint8_t* current_data_;
   unsigned current_data_offset_;
   unsigned current_data_length_;
   // For converting UTF-8 characters which are split across two data chunks.
@@ skipping 25 matching lines @@
     // Entire string is read at start.
     return false;
   }
   Handle<ExternalTwoByteString> source_;
   const uc16* raw_data_;  // Pointer to the actual array of characters.
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_SCANNER_CHARACTER_STREAMS_H_