Replace NULL with nullptr in all C++ files.

webrtc/base/platform_thread.cc

Index: webrtc/base/platform_thread.cc
diff --git a/webrtc/base/platform_thread.cc b/webrtc/base/platform_thread.cc
index 0b025f2af048f94d9e776f171276850f54ef952e..407e199f4c320ab503577bb69264b3fe0747daa0 100644
--- a/webrtc/base/platform_thread.cc
+++ b/webrtc/base/platform_thread.cc
@@ -99,7 +99,7 @@ PlatformThread::PlatformThread(ThreadRunFunction func,
       name_(thread_name ? thread_name : "webrtc"),
 #if defined(WEBRTC_WIN)
       stop_(false),
-      thread_(NULL),
+      thread_(nullptr),
       thread_id_(0) {
 #else
       stop_event_(false, false),
@@ -143,7 +143,7 @@ void PlatformThread::Start() {
   // See bug 2902 for background on STACK_SIZE_PARAM_IS_A_RESERVATION.
   // Set the reserved stack stack size to 1M, which is the default on Windows
   // and Linux.
-  thread_ = ::CreateThread(NULL, 1024 * 1024, &StartThread, this,
+  thread_ = ::CreateThread(nullptr, 1024 * 1024, &StartThread, this,
                            STACK_SIZE_PARAM_IS_A_RESERVATION, &thread_id_);
   RTC_CHECK(thread_) << "CreateThread failed";
   RTC_DCHECK(thread_id_);