Add Foundation.framework dependency to blink_platform

Right now, blink_platform is inheriting this link_setting from 'wtf',
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.

BUG=

Review URL: https://codereview.chromium.org/26680003

git-svn-id: svn://svn.chromium.org/blink/trunk@159244 bbb929c8-8fbe-4397-9dbb-9b2b20218538