Replace NULL with nullptr in all C++ files.

webrtc/base/macconversion.cc

Index: webrtc/base/macconversion.cc
diff --git a/webrtc/base/macconversion.cc b/webrtc/base/macconversion.cc
index c1eec03de75b3a540d45282bd973d236e90207c8..d8783266ff5b01fd7753715d2f44146326e276e9 100644
--- a/webrtc/base/macconversion.cc
+++ b/webrtc/base/macconversion.cc
@@ -20,7 +20,7 @@ bool p_convertHostCFStringRefToCPPString(
   bool result = false;
 
   // First this must be non-null,
-  if (NULL != cfstr) {
+  if (nullptr != cfstr) {
     // it must actually *be* a CFString, and not something just masquerading
     // as one,
     if (CFGetTypeID(cfstr) == CFStringGetTypeID()) {
@@ -47,7 +47,7 @@ bool p_convertCFNumberToInt(CFNumberRef cfn, int* i) {
   bool converted = false;
 
   // It must not be null.
-  if (NULL != cfn) {
+  if (nullptr != cfn) {
     // It must actually *be* a CFNumber and not something just masquerading
     // as one.
     if (CFGetTypeID(cfn) == CFNumberGetTypeID()) {