Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/cpu_info.cc

Index: webrtc/system_wrappers/source/cpu_info.cc
diff --git a/webrtc/system_wrappers/source/cpu_info.cc b/webrtc/system_wrappers/source/cpu_info.cc
index 4a61c7a6f3a00e9fdaf4af4235fb7fce4e98b7de..67f7dc2040c1607f2bfd4489bdc0c312d3eb2f86 100644
--- a/webrtc/system_wrappers/source/cpu_info.cc
+++ b/webrtc/system_wrappers/source/cpu_info.cc
@@ -39,7 +39,7 @@ static int DetectNumberOfCores() {
 #elif defined(WEBRTC_MAC)
   int name[] = {CTL_HW, HW_AVAILCPU};
   size_t size = sizeof(number_of_cores);
-  if (0 != sysctl(name, 2, &number_of_cores, &size, NULL, 0)) {
+  if (0 != sysctl(name, 2, &number_of_cores, &size, nullptr, 0)) {
     LOG(LS_ERROR) << "Failed to get number of cores";
     number_of_cores = 1;
   }