hide discardable factory from public imagegenerator api

BUG=skia:
R=halcanary@google.com, scroggo@google.com, djsollen@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/295243006

git-svn-id: http://skia.googlecode.com/svn/trunk@14889 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/samplecode/SampleEncode.cpp b/samplecode/SampleEncode.cpp
index 080c05f..e65b2d3 100644
--- a/samplecode/SampleEncode.cpp
+++ b/samplecode/SampleEncode.cpp
@@ -196,7 +196,7 @@
                     if (!SkInstallDiscardablePixelRef(
                             SkDecodingImageGenerator::Create(encoded,
                                 SkDecodingImageGenerator::Options()),
-                            &bm, NULL)) {
+                            &bm)) {
                     SkDebugf("[%s:%d] failed to decode %s%s\n",
                              __FILE__, __LINE__,gConfigLabels[i], gExt[j]);
                     }