GTMLocalizer: use NSAccessibility on 10.10+ deployment targets

This change makes GTMLocalizer use NSAccessibility when building for 10.10+
deployment targets.

I tested this change by:
1) Hacking the GTM Xcode project to build with a 10.10 deployment target, and
   ignoring the other deprecated API uses;
2) Patching this GTM into the chromium tree (which does target 10.10) and
   ensuring that it still builds