Add Alpha8 support to SkPNGImageEncoder

tests/CodecTest.cpp

Index: tests/CodecTest.cpp
diff --git a/tests/CodecTest.cpp b/tests/CodecTest.cpp
index fcbfadd0680bc25225fd1f0bb1f94d85dc779a60..09264181bee1f81741d0ce3cedb6139501650f1b 100644
--- a/tests/CodecTest.cpp
+++ b/tests/CodecTest.cpp
@@ -1119,4 +1119,16 @@ DEF_TEST(Codec_PngRoundTrip, r) {
     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);
 }