Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/rw_lock_win.cc

Index: webrtc/system_wrappers/source/rw_lock_win.cc
diff --git a/webrtc/system_wrappers/source/rw_lock_win.cc b/webrtc/system_wrappers/source/rw_lock_win.cc
index 2372b9b50396cf97148f0fc6263bb07736768f58..cd85b49f73e45af5a3305961bd0f93c65aec89a6 100644
--- a/webrtc/system_wrappers/source/rw_lock_win.cc
+++ b/webrtc/system_wrappers/source/rw_lock_win.cc
@@ -16,7 +16,7 @@ namespace webrtc {
 
 static bool native_rw_locks_supported = false;
 static bool module_load_attempted = false;
-static HMODULE library = NULL;
+static HMODULE library = nullptr;
 
 typedef void (WINAPI* InitializeSRWLock)(PSRWLOCK);
 
@@ -38,7 +38,7 @@ RWLockWin::RWLockWin() {
 
 RWLockWin* RWLockWin::Create() {
   if (!LoadModule()) {
-    return NULL;
+    return nullptr;
   }
   return new RWLockWin();
 }