Remove WebKit.grd all together with Source/WebKit/ directory.

Initially WebKit.grd copied into public/web/blink_resources.grd and
Chromium was migrated to use it. Now that all changes to migrate to
blink_resources.grd were done (both on Blink side and Chromium side),
we can remove this file now. We can also remove this directory because all
the relevant files were already moved (Adam's changes) elsewhere in tree,
and the remaining files are useless.

R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21847002

git-svn-id: svn://svn.chromium.org/blink/trunk@155456 bbb929c8-8fbe-4397-9dbb-9b2b20218538