IWYU SkPixelSerializer is no longer brought in by these other skia headers

Needed to land https://skia-review.googlesource.com/c/skia/+/76980

Bug: 
Change-Id: I07f1647b0d342baebadfa0549f54e5252ee4395d
Reviewed-on: https://chromium-review.googlesource.com/809089Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521755}

skia/ext/skia_utils_base.cc

@@ -9,6 +9,7 @@
 #include "base/pickle.h"
 #include "third_party/skia/include/core/SkData.h"
 #include "third_party/skia/include/core/SkImageDeserializer.h"
+#include "third_party/skia/include/core/SkPixelSerializer.h"
 #include "third_party/skia/include/core/SkWriteBuffer.h"
 #include "third_party/skia/src/core/SkValidatingReadBuffer.h"