Option for SkCodec to treat dst as all zeroes.

This recreates SkImageDecoder's feature to skip writing zeroes for
SkCodec.

Review URL: https://codereview.chromium.org/980903002
diff --git a/tests/CachedDecodingPixelRefTest.cpp b/tests/CachedDecodingPixelRefTest.cpp
index 3882026..d6edda7 100644
--- a/tests/CachedDecodingPixelRefTest.cpp
+++ b/tests/CachedDecodingPixelRefTest.cpp
@@ -190,6 +190,7 @@
     }
 
     virtual Result onGetPixels(const SkImageInfo& info, void* pixels, size_t rowBytes,
+                               const Options&,
                                SkPMColor ctable[], int* ctableCount) SK_OVERRIDE {
         REPORTER_ASSERT(fReporter, pixels != NULL);
         REPORTER_ASSERT(fReporter, rowBytes >= info.minRowBytes());