Fix png encoding in skia debugger

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2119513002

Review-Url: https://codereview.chromium.org/2119513002
diff --git a/tools/debugger/SkDrawCommand.h b/tools/debugger/SkDrawCommand.h
index 0d1ece0..8b8ada2 100644
--- a/tools/debugger/SkDrawCommand.h
+++ b/tools/debugger/SkDrawCommand.h
@@ -61,7 +61,7 @@
     static const int kOpTypeCount = kLast_OpType + 1;
 
     static void WritePNG(const png_bytep rgba, png_uint_32 width, png_uint_32 height,
-                         SkWStream& out);
+                         SkWStream& out, bool isOpaque);
 
     SkDrawCommand(OpType opType);