Replace NULL with nullptr in all C++ files.

webrtc/base/platform_file.cc

Index: webrtc/base/platform_file.cc
diff --git a/webrtc/base/platform_file.cc b/webrtc/base/platform_file.cc
index a41324fcf9cc0a768ade872c06791e66f1f4e281..a00543cc33049fc3abaf1e9b5347af2af7c36c57 100644
--- a/webrtc/base/platform_file.cc
+++ b/webrtc/base/platform_file.cc
@@ -26,10 +26,10 @@ const PlatformFile kInvalidPlatformFileValue = INVALID_HANDLE_VALUE;
 
 FILE* FdopenPlatformFileForWriting(PlatformFile file) {
   if (file == kInvalidPlatformFileValue)
-    return NULL;
+    return nullptr;
   int fd = _open_osfhandle(reinterpret_cast<intptr_t>(file), 0);
   if (fd < 0)
-    return NULL;
+    return nullptr;
 
   return _fdopen(fd, "w");
 }