add runtime option to provide data->imagegenerator factory

BUG=skia:

Review URL: https://codereview.chromium.org/1229933003
diff --git a/tools/LazyDecodeBitmap.cpp b/tools/LazyDecodeBitmap.cpp
index 6c655da..bcd8cad 100644
--- a/tools/LazyDecodeBitmap.cpp
+++ b/tools/LazyDecodeBitmap.cpp
@@ -28,7 +28,7 @@
         return false;
     }
 
-    SkAutoTDelete<SkImageGenerator> gen(SkImageGenerator::NewFromData(data));
+    SkAutoTDelete<SkImageGenerator> gen(SkImageGenerator::NewFromEncoded(data));
     if (NULL == gen.get()) {
         return false;
     }