Use demux API in SkWebpCodec

tests/CodecTest.cpp

Index: tests/CodecTest.cpp
diff --git a/tests/CodecTest.cpp b/tests/CodecTest.cpp
index fcbfadd0680bc25225fd1f0bb1f94d85dc779a60..9c60be4b6bf8ac8313297ccf9bf6939fb3c380a9 100644
--- a/tests/CodecTest.cpp
+++ b/tests/CodecTest.cpp
@@ -879,7 +879,7 @@ public:
         return fStream.rewind();
     }
     bool isAtEnd() const override {
-        return false;
+        return fStream.isAtEnd();
     }
 private:
     SkMemoryStream fStream;