Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/desktop_frame.cc

Index: webrtc/modules/desktop_capture/desktop_frame.cc
diff --git a/webrtc/modules/desktop_capture/desktop_frame.cc b/webrtc/modules/desktop_capture/desktop_frame.cc
index dfc0928b6631b83522e498dd07224c1c59e2bf8b..e0016b1ead5e68976788309ce7433dc711a08e81 100644
--- a/webrtc/modules/desktop_capture/desktop_frame.cc
+++ b/webrtc/modules/desktop_capture/desktop_frame.cc
@@ -58,10 +58,10 @@ uint8_t* DesktopFrame::GetFrameDataAtPos(const DesktopVector& pos) const {
 }
 
 BasicDesktopFrame::BasicDesktopFrame(DesktopSize size)
-    : DesktopFrame(size, kBytesPerPixel * size.width(),
+    : DesktopFrame(size,
+                   kBytesPerPixel * size.width(),
                    new uint8_t[kBytesPerPixel * size.width() * size.height()],
-                   NULL) {
-}
+                   nullptr) {}
 
 BasicDesktopFrame::~BasicDesktopFrame() {
   delete[] data_;