Build fix: I put the defines in the wrong section.
Build fix: I put the defines in the wrong section.

When building with use_system_libpng, the build will end up using the
png.h from third_party because I put the defines stanza in the wrong
place.

(reported by fta)


git-svn-id: http://src.chromium.org/svn/trunk/src@23561 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: fd522425549a5436254a1a96a35e1c89fbf54fb2
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser