Remove unused WorkerGlobalScope::m_domURL field.

The URL constructor is now available via the generated bindings.

R=mkwst@chromium.org
BUG=

Review URL: https://codereview.chromium.org/176553003

git-svn-id: svn://svn.chromium.org/blink/trunk@167647 bbb929c8-8fbe-4397-9dbb-9b2b20218538