libchromeos: Replace NULL with nullptr

BUG=None
TEST=FEATURES=test emerge-link libchromeos

Change-Id: I41c74e364a8299e148db3e81e5146755cc3e45ea
Reviewed-on: https://chromium-review.googlesource.com/214833
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/chromeos/asynchronous_signal_handler.cc b/chromeos/asynchronous_signal_handler.cc
index 42e8771..2896f08 100644
--- a/chromeos/asynchronous_signal_handler.cc
+++ b/chromeos/asynchronous_signal_handler.cc
@@ -34,7 +34,7 @@
       PLOG(WARNING) << "Failed to close file descriptor";
 
     descriptor_ = kInvalidDescriptor;
-    CHECK_EQ(0, sigprocmask(SIG_SETMASK, &saved_signal_mask_, NULL));
+    CHECK_EQ(0, sigprocmask(SIG_SETMASK, &saved_signal_mask_, nullptr));
   }
 }
 
@@ -95,8 +95,8 @@
 
 void AsynchronousSignalHandler::UpdateSignals() {
   if (descriptor_ != kInvalidDescriptor) {
-    CHECK_EQ(0, sigprocmask(SIG_SETMASK, &saved_signal_mask_, NULL));
-    CHECK_EQ(0, sigprocmask(SIG_BLOCK, &signal_mask_, NULL));
+    CHECK_EQ(0, sigprocmask(SIG_SETMASK, &saved_signal_mask_, nullptr));
+    CHECK_EQ(0, sigprocmask(SIG_BLOCK, &signal_mask_, nullptr));
     CHECK_EQ(descriptor_,
              signalfd(descriptor_, &signal_mask_, SFD_CLOEXEC | SFD_NONBLOCK));
   }