Move WebCore::shutdown into the CoreInitializer class

Before this CL, this function oddly dangled outside the class that manages the
state it manipulates.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/393983003

git-svn-id: svn://svn.chromium.org/blink/trunk@178215 bbb929c8-8fbe-4397-9dbb-9b2b20218538