add runtime option to provide data->imagegenerator factory

gm/factory.cpp

Index: gm/factory.cpp
diff --git a/gm/factory.cpp b/gm/factory.cpp
index 4af37636f9d647b027d389733d4d06a4a156b5b4..0a938629b9538b33da8cf14de2398d7fbbca160c 100644
--- a/gm/factory.cpp
+++ b/gm/factory.cpp
@@ -36,7 +36,7 @@ protected:
             // bitmap is unlocked.
             SkAutoTUnref<SkDiscardableMemoryPool> pool(
                 SkDiscardableMemoryPool::Create(1));
-            SkAssertResult(SkInstallDiscardablePixelRef(SkImageGenerator::NewFromData(data),
+            SkAssertResult(SkInstallDiscardablePixelRef(SkImageGenerator::NewFromEncoded(data),
                                                         NULL, &fBitmap, pool));
         }
     }