add runtime option to provide data->imagegenerator factory

BUG=skia:

Review URL: https://codereview.chromium.org/1229933003
diff --git a/tests/CachedDecodingPixelRefTest.cpp b/tests/CachedDecodingPixelRefTest.cpp
index a2032ba..78adc34d 100644
--- a/tests/CachedDecodingPixelRefTest.cpp
+++ b/tests/CachedDecodingPixelRefTest.cpp
@@ -142,7 +142,7 @@
 
 ////////////////////////////////////////////////////////////////////////////////
 static bool install_skCachingPixelRef(SkData* encoded, SkBitmap* dst) {
-    return SkCachingPixelRef::Install(SkImageGenerator::NewFromData(encoded), dst);
+    return SkCachingPixelRef::Install(SkImageGenerator::NewFromEncoded(encoded), dst);
 }
 static bool install_skDiscardablePixelRef(SkData* encoded, SkBitmap* dst) {
     // Use system-default discardable memory.