Make SkImageDecoder_libpng be compilable with Chromium libpng

Chromium has libpng with pngusr.h that disables certain functionality with
defines. Use those defines when compiling SkImageDecoder_libpng.cpp.

This makes it possible to use Skia PNG decoder and encoder when compiling dm
with Chromium. Skia PNG decoder and encoder of course is not compiled to the
skia library, but to dm with Chromium.

BUG=skia:2992

Review URL: https://codereview.chromium.org/645883004