Created attachment 429736 [details, diff] patch for adding accessibility USE flag webkit-gtk (at least 2.10.7 that I tested on) supports being built without accessibility support. This patch adds the accessibility USE flag to the ebuild.
Created attachment 429738 [details] resulting ebuild
It looks like they are used at runtime: http://mariospr.org/2013/02/03/accessibility-in-webkitgtk/ Also, looking to the sources, atk looks to be indeed used (and doesn't look an optional component with any switch to disable it at compile time) and all the upstream tools at Tools/gtk/ are also ensuring that deps to be installed :/