Add scanline decoding to SkCodec.

src/codec/SkCodec.cpp

Index: src/codec/SkCodec.cpp
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index 0411e44069e32cd15e7e218ac2100eb6649e6080..015b1229747f7e7aec98c21062ecf4cf6e7a31e7 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -61,3 +61,12 @@ bool SkCodec::rewindIfNeeded() {
     fNeedsRewind = true;
     return !neededRewind || fStream->rewind();
 }
+
+SkScanlineDecoder* SkCodec::getScanlineDecoder(const SkImageInfo& dstInfo) {
+    fScanlineDecoder.reset(NULL);
+    if (!rewindIfNeeded()) {
+        return NULL;
+    }
+    fScanlineDecoder.reset(this->onGetScanlineDecoder(dstInfo));
+    return fScanlineDecoder.get();
+}