Start writing blink platform GN file.

This adds a target for blink_common, and starts a rough skeleton of blink_platform. There are a lot of dependencies for the platform pat (like V8) so I thought it best to checkpoint that before I start worrying about those.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/257733002

git-svn-id: svn://svn.chromium.org/blink/trunk@172998 bbb929c8-8fbe-4397-9dbb-9b2b20218538