Replace NULL with nullptr in all C++ files.

webrtc/tools/psnr_ssim_analyzer/psnr_ssim_analyzer.cc

Index: webrtc/tools/psnr_ssim_analyzer/psnr_ssim_analyzer.cc
diff --git a/webrtc/tools/psnr_ssim_analyzer/psnr_ssim_analyzer.cc b/webrtc/tools/psnr_ssim_analyzer/psnr_ssim_analyzer.cc
index 737661c3df0838ddb6d73653f2ccd7d42b1a4219..480a8a3ade483a9a3f0f9dfb5ebe83d5e65b4aa1 100644
--- a/webrtc/tools/psnr_ssim_analyzer/psnr_ssim_analyzer.cc
+++ b/webrtc/tools/psnr_ssim_analyzer/psnr_ssim_analyzer.cc
@@ -119,8 +119,8 @@ int main(int argc, char** argv) {
   }
   parser.PrintEnteredFlags();
 
-  int width = strtol((parser.GetFlag("width")).c_str(), NULL, 10);
-  int height = strtol((parser.GetFlag("height")).c_str(), NULL, 10);
+  int width = strtol((parser.GetFlag("width")).c_str(), nullptr, 10);
+  int height = strtol((parser.GetFlag("height")).c_str(), nullptr, 10);
 
   if (width <= 0 || height <= 0) {
     fprintf(stderr, "Error: width or height cannot be <= 0!\n");