[Android] Fix gn build for instrumentation tests.

tools/android/forwarder2/forwarders_manager.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/android/forwarder2/forwarders_manager.h"
 
 #include <sys/select.h>
 #include <unistd.h>
 
 #include <algorithm>
@@ skipping 63 matching lines @@
        it != forwarders_.end(); ++it) {
     Forwarder* const forwarder = *it;
     forwarder->RegisterFDs(&read_fds, &write_fds, &max_fd);
   }
 
   const int notifier_fds[] = {
     wakeup_notifier_.receiver_fd(),
     deletion_notifier_.receiver_fd(),
   };
 
-  for (int i = 0; i < arraysize(notifier_fds); ++i) {
+  for (size_t i = 0; i < arraysize(notifier_fds); ++i) {
     const int notifier_fd = notifier_fds[i];
     DCHECK_GT(notifier_fd, -1);
     FD_SET(notifier_fd, &read_fds);
     max_fd = std::max(max_fd, notifier_fd);
   }
 
   const int ret = HANDLE_EINTR(
       select(max_fd + 1, &read_fds, &write_fds, NULL, NULL));
   if (ret < 0) {
     PLOG(ERROR) << "select";
@@ skipping 28 matching lines @@
       ++i;
       continue;
     }
 
     std::swap(forwarders_[i], forwarders_.back());
     forwarders_.pop_back();
   }
 }
 
 }  // namespace forwarder2