Fix scoped_refptr construction from NULL

This is a precursor to adding a new scoped_refptr(std::nullptr_t)
constructor. The implicit conversion from NULL to scoped_refptr<T>
causes a compilation error with the new constructor. Replace NULL with
nullptr in any files where this is a problem.

This CL was uploaded by git cl split.

R=liberato@chromium.org

Bug: 1018887
Change-Id: I589a654b9f76a8e066533ea789fdd9c6f7bb3c14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885079
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710337}

diff --git a/media/filters/decrypting_audio_decoder_unittest.cc b/media/filters/decrypting_audio_decoder_unittest.cc
index cd936d3..1932020 100644
--- a/media/filters/decrypting_audio_decoder_unittest.cc
+++ b/media/filters/decrypting_audio_decoder_unittest.cc

@@ -64,7 +64,7 @@
         num_decrypt_and_decode_calls_(0),
         num_frames_in_decryptor_(0),
         encrypted_buffer_(CreateFakeEncryptedBuffer()),
-        decoded_frame_(NULL),
+        decoded_frame_(nullptr),
         decoded_frame_list_() {}
 
   ~DecryptingAudioDecoderTest() override { Destroy(); }