change pixel-serializer to support reencoding existing data

src/core/SkPixelSerializer.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkPixelSerializer.h"
+
+SkData* SkPixelSerializer::reencodeData(SkData* encoded) {
+    return encoded ? this->onReencodeData(encoded) : nullptr;
+}
+
+SkData* SkPixelSerializer::encodePixels(const SkImageInfo& info, const void* pixels,
+                                        size_t rowBytes) {
+    if (kUnknown_SkColorType == info.colorType() || !pixels) {
+        return nullptr;
+    }
+    return this->onEncodePixels(info, pixels, rowBytes);
+}
+
+SkData* SkPixelSerializer::encodePixels(const SkPixmap& pixmap) {
+    return this->encodePixels(pixmap.info(), pixmap.addr(), pixmap.rowBytes());
+}