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

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2718663005
Cr-Commit-Position: refs/heads/master@{#16878}
diff --git a/webrtc/base/stringencode.cc b/webrtc/base/stringencode.cc
index 7c11a05..c7d206f 100644
--- a/webrtc/base/stringencode.cc
+++ b/webrtc/base/stringencode.cc
@@ -141,7 +141,7 @@
 
 size_t url_encode(char * buffer, size_t buflen,
                   const char * source, size_t srclen) {
-  if (NULL == buffer)
+  if (nullptr == buffer)
     return srclen * 3 + 1;
   if (buflen <= 0)
     return 0;
@@ -167,7 +167,7 @@
 
 size_t url_decode(char * buffer, size_t buflen,
                   const char * source, size_t srclen) {
-  if (NULL == buffer)
+  if (nullptr == buffer)
     return srclen + 1;
   if (buflen <= 0)
     return 0;
@@ -546,7 +546,7 @@
 
 std::string s_transform(const std::string& source, Transform t) {
   // Ask transformation function to approximate the destination size (returns upper bound)
-  size_t maxlen = t(NULL, 0, source.data(), source.length());
+  size_t maxlen = t(nullptr, 0, source.data(), source.length());
   char * buffer = STACK_ARRAY(char, maxlen);
   size_t len = t(buffer, maxlen, source.data(), source.length());
   std::string result(buffer, len);