Replace NULL with nullptr or null in webrtc/api/.

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2715103002
Cr-Commit-Position: refs/heads/master@{#16880}
diff --git a/webrtc/api/jsepicecandidate.h b/webrtc/api/jsepicecandidate.h
index a8919e0..3566013 100644
--- a/webrtc/api/jsepicecandidate.h
+++ b/webrtc/api/jsepicecandidate.h
@@ -31,7 +31,7 @@
   JsepIceCandidate(const std::string& sdp_mid, int sdp_mline_index,
                    const cricket::Candidate& candidate);
   ~JsepIceCandidate();
-  // |err| may be NULL.
+  // |err| may be null.
   bool Initialize(const std::string& sdp, SdpParseError* err);
   void SetCandidate(const cricket::Candidate& candidate) {
     candidate_ = candidate;