Fix warnings on Win64.

src/log-utils.h

 // Copyright 2006-2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 111 matching lines @@
 
   // Initialization function called from Open... functions.
   static void Init();
 
   // Write functions assume that mutex_ is acquired by the caller.
   static WritePtr Write;
 
   // Implementation of writing to a log file.
   static int WriteToFile(const char* msg, int length) {
     ASSERT(output_handle_ != NULL);
-    int rv = fwrite(msg, 1, length, output_handle_);
-    ASSERT(length == rv);
-    return rv;
+    size_t rv = fwrite(msg, 1, length, output_handle_);
+    ASSERT(static_cast<size_t>(length) == rv);
+    USE(rv);
+    return length;
   }
 
   // Implementation of writing to a memory buffer.
   static int WriteToMemory(const char* msg, int length) {
     ASSERT(output_buffer_ != NULL);
     return output_buffer_->Write(msg, length);
   }
 
   // Whether logging is stopped (e.g. due to insufficient resources).
   static bool is_stopped_;
@@ skipping 137 matching lines @@
 
   ScopedLock sl;
   int pos_;
 };
 
 #endif  // ENABLE_LOGGING_AND_PROFILING
 
 } }  // namespace v8::internal
 
 #endif  // V8_LOG_UTILS_H_