Replace NULL with nullptr in all C++ files.

webrtc/pc/jsepsessiondescription.cc

Index: webrtc/pc/jsepsessiondescription.cc
diff --git a/webrtc/pc/jsepsessiondescription.cc b/webrtc/pc/jsepsessiondescription.cc
index 02919b6bf8b7328e7fd99338815a5ba205af1158..1b1da3bd8ab392ddaed64d4b36b643f6e60541b5 100644
--- a/webrtc/pc/jsepsessiondescription.cc
+++ b/webrtc/pc/jsepsessiondescription.cc
@@ -49,13 +49,13 @@ SessionDescriptionInterface* CreateSessionDescription(const std::string& type,
                                                       const std::string& sdp,
                                                       SdpParseError* error) {
   if (!IsTypeSupported(type)) {
-    return NULL;
+    return nullptr;
   }
 
   JsepSessionDescription* jsep_desc = new JsepSessionDescription(type);
   if (!jsep_desc->Initialize(sdp, error)) {
     delete jsep_desc;
-    return NULL;
+    return nullptr;
   }
   return jsep_desc;
 }
@@ -146,7 +146,7 @@ size_t JsepSessionDescription::number_of_mediasections() const {
 const IceCandidateCollection* JsepSessionDescription::candidates(
     size_t mediasection_index) const {
   if (mediasection_index >= candidate_collection_.size())
-    return NULL;
+    return nullptr;
   return &candidate_collection_[mediasection_index];
 }