Fix linux/gcc compiler warnings in tools/ipc_fuzzer.

tools/ipc_fuzzer/message_lib/message_file_writer.cc

Index: tools/ipc_fuzzer/message_lib/message_file_writer.cc
diff --git a/tools/ipc_fuzzer/message_lib/message_file_writer.cc b/tools/ipc_fuzzer/message_lib/message_file_writer.cc
index 46953d8f5004513c47f56bdb3f44482da820d077..2bdc9ae30f70dd8a8796b1507929bfb178e9367f 100644
--- a/tools/ipc_fuzzer/message_lib/message_file_writer.cc
+++ b/tools/ipc_fuzzer/message_lib/message_file_writer.cc
@@ -79,7 +79,7 @@ bool Writer::WriteBlob(const void *buffer, size_t size) {
     return false;
   const char* char_buffer = static_cast<const char*>(buffer);
   int ret = base::WritePlatformFileAtCurrentPos(file_, char_buffer, size);
-  if (ret != size) {
+  if (ret != static_cast<int>(size)) {
     LOG(ERROR) << "Failed to write " << size << " bytes.";
     return false;
   }