GN: Forward icu_config from public header config since we expose icu

public/web/WebSettings.h does:
and uses some types from ICU in its definition. Thus, anything that
depends on the blink public API and includes this header must have the
icu include dirs on their include path. This forwards icu_config from
the blink_headers target so dependents do not have to do this themselves
(unless they are linking against ICU directly)

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/563963003

git-svn-id: svn://svn.chromium.org/blink/trunk@181876 bbb929c8-8fbe-4397-9dbb-9b2b20218538