Make SkPngCodec decode progressively.

src/codec/SkIcoCodec.h

Index: src/codec/SkIcoCodec.h
diff --git a/src/codec/SkIcoCodec.h b/src/codec/SkIcoCodec.h
index 8c56aee4c6ca135da33d0aa7824db1fab863bd84..988c8b6ff765c1def632a8bd337544b5ed0f6419 100644
--- a/src/codec/SkIcoCodec.h
+++ b/src/codec/SkIcoCodec.h
@@ -54,6 +54,12 @@ private:
 
     bool onSkipScanlines(int count) override;
 
+    Result onStartIncrementalDecode(const SkImageInfo& dstInfo,
+            const SkCodec::Options&, SkPMColor*, int*) override;
+
+    Result onIncrementalDecode(std::function<void*(int)> callback,
+                               int* rowsDecoded) override;
+
     SkSampler* getSampler(bool createIfNecessary) override;
 
     /*
@@ -84,5 +90,13 @@ private:
     // SkAutoTDelete.  It will be deleted by the destructor of fEmbeddedCodecs.
     SkCodec* fCurrScanlineCodec;
 
+    // Only used by incremental decoder.  onStartIncrementalDecode() will set
+    // fCurrIncrementalCodec to one of the fEmbeddedCodecs, if it can find a
+    // codec of the appropriate size.  We will use fCurrIncrementalCodec for
+    // subsequent calls to incrementalDecode().
+    // fCurrIncrementalCodec is owned by this class, but should not be an
+    // SkAutoTDelete.  It will be deleted by the destructor of fEmbeddedCodecs.
+    SkCodec* fCurrIncrementalCodec;
+
     typedef SkCodec INHERITED;
 };