hide discardable factory from public imagegenerator api

tests/ImageDecodingTest.cpp

Index: tests/ImageDecodingTest.cpp
diff --git a/tests/ImageDecodingTest.cpp b/tests/ImageDecodingTest.cpp
index d5dd11ac85fdb39ac5028b105a833c34dfb27842..31327d6bc257d1da6aa41784392882e17e6fbdd3 100644
--- a/tests/ImageDecodingTest.cpp
+++ b/tests/ImageDecodingTest.cpp
@@ -16,7 +16,7 @@
 #include "SkGradientShader.h"
 #include "SkImageDecoder.h"
 #include "SkImageEncoder.h"
-#include "SkImageGenerator.h"
+#include "SkImageGeneratorPriv.h"
 #include "SkImagePriv.h"
 #include "SkOSFile.h"
 #include "SkPoint.h"
@@ -455,7 +455,7 @@ DEF_TEST(WebP, reporter) {
 
     bool success = SkInstallDiscardablePixelRef(
         SkDecodingImageGenerator::Create(encoded,
-            SkDecodingImageGenerator::Options()), &bm, NULL);
+            SkDecodingImageGenerator::Options()), &bm);
 
     REPORTER_ASSERT(reporter, success);
     if (!success) {
@@ -594,14 +594,13 @@ static void test_options(skiatest::Reporter* reporter,
             return;
         }
         success = SkInstallDiscardablePixelRef(
-            SkDecodingImageGenerator::Create(encodedData, opts), &bm, NULL);
+            SkDecodingImageGenerator::Create(encodedData, opts), &bm);
     } else {
         if (NULL == encodedStream) {
             return;
         }
         success = SkInstallDiscardablePixelRef(
-            SkDecodingImageGenerator::Create(encodedStream->duplicate(), opts),
-            &bm, NULL);
+            SkDecodingImageGenerator::Create(encodedStream->duplicate(), opts), &bm);
     }
     if (!success) {
         if (opts.fUseRequestedColorType