Replace NULL with nullptr in all C++ files.

webrtc/api/statstypes.cc

Index: webrtc/api/statstypes.cc
diff --git a/webrtc/api/statstypes.cc b/webrtc/api/statstypes.cc
index 300bb4a9339711608a70aef3b7a14f6e079ba869..b7459fa1fa4a58865f8e80bdb0dc7baada2a80d3 100644
--- a/webrtc/api/statstypes.cc
+++ b/webrtc/api/statstypes.cc
@@ -779,7 +779,7 @@ StatsReport* StatsCollection::ReplaceOrAddNew(const StatsReport::Id& id) {
   return InsertNew(id);
 }
 
-// Looks for a report with the given |id|.  If one is not found, NULL
+// Looks for a report with the given |id|.  If one is not found, null
 // will be returned.
 StatsReport* StatsCollection::Find(const StatsReport::Id& id) {
   RTC_DCHECK(thread_checker_.CalledOnValidThread());