Introduce public/blink_skia_config.gyp

This CL copies Source/WebKit/chromium/skia_webkit.gyp into the public directory
(and renames it to blink_skia_config.gyp). This GYP file is referenced directly
from Chromium, which means it should be in the public directory. We're about to
remove Source/WebKit/chromium, which means this file needs to move sooner
rather than later.

R=jamesr@chromium.org, tfarina@chromium.org

Review URL: https://codereview.chromium.org/21137002

git-svn-id: svn://svn.chromium.org/blink/trunk@155116 bbb929c8-8fbe-4397-9dbb-9b2b20218538