DEPS libpng into third_party/libpng/src, instead if third_party/libpng.
Then we'll be able to put its BUILD.gn into third_party/libpng instead
of the more distant build/secondary/... location.

BUG=angleproject:1569

Change-Id: I12b1c51d65682cba3954c26f1bd3f7491ac31b01
Reviewed-on: https://chromium-review.googlesource.com/722176
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>