Revert "Move WebKit.gyp out of Source/WebKit/chromium"

This reverts r155175. It appears to have broken the runhooks step on
some of the bots.

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/21004012

git-svn-id: svn://svn.chromium.org/blink/trunk@155176 bbb929c8-8fbe-4397-9dbb-9b2b20218538