Add dependency on Carbon.framework from webcore_platform

Since webcore_platform is using symbols from Carbon.framework, it
should specify this in link_settings.

Right now, libwebkit is inheriting this link_setting from 'base', but
this is taking advantage of a GYP bug where link_settings were
propagated outside shared_library targets.

The GYP bug is fixed in r1717, but in order to enable it, Blink's
link_settings need to be fixed first.

R=mark@chromium.org
BUG=

Review URL: https://codereview.chromium.org/25915003

git-svn-id: svn://svn.chromium.org/blink/trunk@158861 bbb929c8-8fbe-4397-9dbb-9b2b20218538