Cleanup WebWorkerBase::dispatchTaskToMainThread usage with WeakPtr and WTF::Functional

BUG=none

Review URL: https://codereview.chromium.org/45223006

git-svn-id: svn://svn.chromium.org/blink/trunk@161160 bbb929c8-8fbe-4397-9dbb-9b2b20218538