Add Alpha8 support to SkPNGImageEncoder

tests/CodecTest.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Resources.h"
 #include "SkAndroidCodec.h"
 #include "SkBitmap.h"
@@ skipping 1101 matching lines @@
 
     // Create an arbitrary gray bitmap.
     path = "grayscale.jpg";
     stream.reset(resource(path));
     codec.reset(SkCodec::NewFromStream(stream.release()));
     SkBitmap bm2;
     bm2.allocPixels(codec->getInfo());
     result = codec->getPixels(codec->getInfo(), bm2.getPixels(), bm2.rowBytes());
     REPORTER_ASSERT(r, SkCodec::kSuccess == result);
     check_round_trip(r, bm2);
+
+    // Create an arbitrary alpha8 bitmap.
+    path = "arrow.png";
+    stream.reset(resource(path));
+    codec.reset(SkCodec::NewFromStream(stream.release()));
+    SkBitmap tmp;
+    tmp.allocPixels(codec->getInfo());
+    result = codec->getPixels(codec->getInfo(), tmp.getPixels(), tmp.rowBytes());
+    REPORTER_ASSERT(r, SkCodec::kSuccess == result);
+    SkBitmap bm3;
+    tmp.copyTo(&bm3, kAlpha_8_SkColorType);
+    check_round_trip(r, bm3);
 }