Move WebKit.gyp out of Source/WebKit/chromium

We're very close to removing Source/WebKit/chromium. This CL moves WebKit.gyp
to Source/web/web.gyp.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/21131004

git-svn-id: svn://svn.chromium.org/blink/trunk@155175 bbb929c8-8fbe-4397-9dbb-9b2b20218538