Make it possible to enable blink_gc_plugin on mac.

We were missing the xcode_settings OTHER_CFLAGS.

R=oilpan-reviews@chromium.org, tkent@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/214403002

git-svn-id: svn://svn.chromium.org/blink/trunk@170222 bbb929c8-8fbe-4397-9dbb-9b2b20218538