Reland "Promote platform exclusion rules to config.gyp."

This includes a fix for Aura build.

BUG=None
TEST=blink_platform_unittests
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/27746006

git-svn-id: svn://svn.chromium.org/blink/trunk@160011 bbb929c8-8fbe-4397-9dbb-9b2b20218538