Implement main blink target in GN.

This adds the "blink" and "blink_minimal" targets which code in Chrome generally uses to depend on Blink. These just forward to already-existing subtargets.

Adds visibility to other BUILD.gn files in blink to mark all non-public files visible only to either WebKit/* or WebKit/Source/*

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/329993006

git-svn-id: svn://svn.chromium.org/blink/trunk@176546 bbb929c8-8fbe-4397-9dbb-9b2b20218538