Fix integer overflow in Windows shared memory handling.

BUG=164490
Review URL: https://codereview.chromium.org/11450016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@171369 0039d316-1c4b-4281-b951-d872f2087c98
void SharedMemory::Unlock() {
  DCHECK(lock_ != NULL);
  ReleaseMutex(lock_);
}

void SharedMemory::Unlock() {
  DCHECK(lock_ != NULL);
  ReleaseMutex(lock_);
}
