Use demux API in SkWebpCodec

* Necessary to read ICC profiles
* Will be necessary to implement animation
* Requires consolidated data with length

Doesn't affect decode performance (thought I believe
our performance tests only cover SkStreams with memory
bases).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2311793004

Review-Url: https://codereview.chromium.org/2311793004
diff --git a/tests/CodecTest.cpp b/tests/CodecTest.cpp
index fcbfadd..9c60be4 100644
--- a/tests/CodecTest.cpp
+++ b/tests/CodecTest.cpp
@@ -879,7 +879,7 @@
         return fStream.rewind();
     }
     bool isAtEnd() const override {
-        return false;
+        return fStream.isAtEnd();
     }
 private:
     SkMemoryStream fStream;