Fix and enable MSVC warning C4018 for base/.

base/synchronization/waitable_event_win.cc

 // Copyright (c) 2011 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 "base/synchronization/waitable_event.h"
 
 #include <windows.h>
 
 #include "base/logging.h"
 #include "base/numerics/safe_conversions.h"
@@ skipping 55 matching lines @@
   // It is most unexpected that this should ever fail.  Help consumers learn
   // about it if it should ever fail.
   NOTREACHED() << "WaitForSingleObject failed";
   return false;
 }
 
 // static
 size_t WaitableEvent::WaitMany(WaitableEvent** events, size_t count) {
   base::ThreadRestrictions::AssertWaitAllowed();
   HANDLE handles[MAXIMUM_WAIT_OBJECTS];
-  CHECK_LE(count, MAXIMUM_WAIT_OBJECTS)
+  CHECK_LE(count, static_cast<size_t>(MAXIMUM_WAIT_OBJECTS))
       << "Can only wait on " << MAXIMUM_WAIT_OBJECTS << " with WaitMany";
 
   for (size_t i = 0; i < count; ++i)
     handles[i] = events[i]->handle();
 
   // The cast is safe because count is small - see the CHECK above.
   DWORD result =
       WaitForMultipleObjects(static_cast<DWORD>(count),
                              handles,
                              FALSE,      // don't wait for all the objects
                              INFINITE);  // no timeout
   if (result >= WAIT_OBJECT_0 + count) {
     DPLOG(FATAL) << "WaitForMultipleObjects failed";
     return 0;
   }
 
   return result - WAIT_OBJECT_0;
 }
 
 }  // namespace base