Fix png encoding in skia debugger

tools/skiaserve/Request.cpp

Index: tools/skiaserve/Request.cpp
diff --git a/tools/skiaserve/Request.cpp b/tools/skiaserve/Request.cpp
index f2f10acad1cf0442d41164efb5bdbf56dd3229a5..6ae6b51b8d95e6054476b2d2f6788ff2c8944d9d 100644
--- a/tools/skiaserve/Request.cpp
+++ b/tools/skiaserve/Request.cpp
@@ -59,11 +59,10 @@ SkData* Request::writeCanvasToPng(SkCanvas* canvas) {
     sk_sp<SkData> encodedBitmap = sk_tools::encode_bitmap_for_png(*bmp);
     SkASSERT(encodedBitmap.get());
 
-    // write to png
+    // write to an opaque png (black background)
     SkDynamicMemoryWStream buffer;
-    SkDrawCommand::WritePNG((const png_bytep) encodedBitmap->writable_data(),
-                            bmp->width(), bmp->height(),
-                            buffer);
+    SkDrawCommand::WritePNG((const png_bytep) encodedBitmap->bytes(), bmp->width(), bmp->height(),
+                            buffer, true);
     return buffer.copyToData();
 }