unified PNG encoding between SkDrawCommand and skiaserve
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1818043003

Review URL: https://codereview.chromium.org/1818043003
diff --git a/gyp/debugger.gyp b/gyp/debugger.gyp
index d6b97a7..f6e1988 100644
--- a/gyp/debugger.gyp
+++ b/gyp/debugger.gyp
@@ -137,6 +137,7 @@
         'debugger_qt_mocs',
         'skia_lib.gyp:skia_lib',
         'jsoncpp.gyp:jsoncpp',
+        'libpng.gyp:libpng',
         'tools.gyp:url_data_manager',
       ],
       'cflags': [