Fix warnings on Win64.

src/log-utils.h

Index: src/log-utils.h
diff --git a/src/log-utils.h b/src/log-utils.h
index 117f098cc8eed9b4b53c55b1f3810590acdaa597..3e25b0e75744fd13a978d053baff14a9de3e308d 100644
--- a/src/log-utils.h
+++ b/src/log-utils.h
@@ -129,9 +129,10 @@ class Log : public AllStatic {
   // 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.