SkPixelSerializer: remove deprecated virtual

Review URL: https://codereview.chromium.org/1512333002
diff --git a/tests/ImageTest.cpp b/tests/ImageTest.cpp
index 2bc2acc..575fc1c 100644
--- a/tests/ImageTest.cpp
+++ b/tests/ImageTest.cpp
@@ -169,7 +169,7 @@
         return false;
     }
 
-    SkData* onEncodePixels(const SkImageInfo&, const void*, size_t) override {
+    SkData* onEncode(const SkPixmap&) override {
         fDidEncode = true;
         return fFunc();
     }