Change WorkerRunLoop::Task to look more similar to WebThread::Task

BUG=301514
R=kinuko@chromium.org

Review URL: https://codereview.chromium.org/179563002

git-svn-id: svn://svn.chromium.org/blink/trunk@167763 bbb929c8-8fbe-4397-9dbb-9b2b20218538