media/video: Rewrite NULL to nullptr

NULL is obsolete and doesn't work with the upcoming CheckedPtr,
so rewrites NULL to nullptr.

Change-Id: If1e2a15c95611157317aac48176d1cdbf3f23251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633487Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844926}

media/video/h264_parser.cc

@@ -306,7 +306,7 @@ H264Parser::H264Parser() {
 H264Parser::~H264Parser() = default;
 
 void H264Parser::Reset() {
-  stream_ = NULL;
+  stream_ = nullptr;
   bytes_left_ = 0;
   encrypted_ranges_.clear();
   previous_nalu_range_.clear();