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/fileutils.cc b/webrtc/base/fileutils.cc
index a883995..ac21c18 100644
--- a/webrtc/base/fileutils.cc
+++ b/webrtc/base/fileutils.cc
@@ -40,7 +40,8 @@
 #ifdef WEBRTC_WIN
     : handle_(INVALID_HANDLE_VALUE) {
 #else
-    : dir_(NULL), dirent_(NULL) {
+    : dir_(nullptr),
+      dirent_(nullptr){
 #endif
 }
 
@@ -68,13 +69,13 @@
   if (handle_ == INVALID_HANDLE_VALUE)
     return false;
 #else
-  if (dir_ != NULL)
+  if (dir_ != nullptr)
     closedir(dir_);
   dir_ = ::opendir(directory_.c_str());
-  if (dir_ == NULL)
+  if (dir_ == nullptr)
     return false;
   dirent_ = readdir(dir_);
-  if (dirent_ == NULL)
+  if (dirent_ == nullptr)
     return false;
 
   if (::stat(std::string(directory_ + Name()).c_str(), &stat_) != 0)
@@ -90,7 +91,7 @@
   return ::FindNextFile(handle_, &data_) == TRUE;
 #else
   dirent_ = ::readdir(dir_);
-  if (dirent_ == NULL)
+  if (dirent_ == nullptr)
     return false;
 
   return ::stat(std::string(directory_ + Name()).c_str(), &stat_) == 0;
@@ -116,7 +117,7 @@
 #endif
 }
 
-FilesystemInterface* Filesystem::default_filesystem_ = NULL;
+FilesystemInterface* Filesystem::default_filesystem_ = nullptr;
 
 FilesystemInterface *Filesystem::EnsureDefaultFilesystem() {
   if (!default_filesystem_) {