gpu/vp8_reference_frame_vector: clean up the header file inclusion

The vp8_reference_frame_vector implementation has dependency to
scoped_refptr.It is cleaner to include the scoped_refptr.h
directly instead of using the ref_counted.h header file.
Cleaning based on the review comments for:
https://chromium-review.googlesource.com/c/chromium/src/+/1383540

Change-Id: I2e97cf4eb8a68690f32148bd6897dd83c56dd213
Reviewed-on: https://chromium-review.googlesource.com/c/1457727Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629887}

media/gpu/vp8_reference_frame_vector.h

@@ -7,7 +7,7 @@
 
 #include <array>
 
-#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_refptr.h"
 #include "base/sequence_checker.h"
 #include "media/filters/vp8_parser.h"