Move blink_headers to separate gyp file

On the iOS build, it is not possible to reference blink.gyp at all as some of
the gyp files referenced by some targets are not checked out in this config.
However, several targets used on iOS do depend on headers exposed by the public
blink API and thus do need to see the blink_headers target. This moves the
blink_headers target into a separate gyp file that can be used universally.

BUG=350097

Review URL: https://codereview.chromium.org/198093005

git-svn-id: svn://svn.chromium.org/blink/trunk@169079 bbb929c8-8fbe-4397-9dbb-9b2b20218538