Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/trace_impl.cc

Index: webrtc/system_wrappers/source/trace_impl.cc
diff --git a/webrtc/system_wrappers/source/trace_impl.cc b/webrtc/system_wrappers/source/trace_impl.cc
index 30a8816b7507eaee376a2ec6c52c1931cb2559f7..a0ccf771f74c9fc745d5f6b3998110dc17bcc428 100644
--- a/webrtc/system_wrappers/source/trace_impl.cc
+++ b/webrtc/system_wrappers/source/trace_impl.cc
@@ -44,7 +44,7 @@ TraceImpl* TraceImpl::StaticInstance(CountOperation count_operation,
   // message will be written to file.
   if ((level != kTraceAll) && (count_operation == kAddRefNoCreate)) {
     if (!(level & level_filter())) {
-      return NULL;
+      return nullptr;
     }
   }
   TraceImpl* impl =
@@ -65,11 +65,10 @@ TraceImpl* TraceImpl::CreateInstance() {
 }
 
 TraceImpl::TraceImpl()
-    : callback_(NULL),
+    : callback_(nullptr),
       row_count_text_(0),
       file_count_text_(0),
-      trace_file_(FileWrapper::Create()) {
-}
+      trace_file_(FileWrapper::Create()) {}
 
 TraceImpl::~TraceImpl() {
   trace_file_->CloseFile();
@@ -326,7 +325,7 @@ int32_t TraceImpl::AddMessage(
   if (written_so_far >= WEBRTC_TRACE_MAX_MESSAGE_SIZE) {
     return -1;
   }
-  // - 2 to leave room for newline and NULL termination.
+// - 2 to leave room for newline and null termination.
 #ifdef _WIN32
   length = _snprintf(trace_message,
                      WEBRTC_TRACE_MAX_MESSAGE_SIZE - written_so_far - 2,
@@ -345,7 +344,7 @@ int32_t TraceImpl::AddMessage(
     trace_message[length] = 0;
   }
 #endif
-  // Length with NULL termination.
+  // Length with null termination.
   return length + 1;
 }