Remove now-unused WorkerThread ExecutionContextTask.

Follow up r171960's removal of unused code and also delete now-unused
task.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/264833007

git-svn-id: svn://svn.chromium.org/blink/trunk@173366 bbb929c8-8fbe-4397-9dbb-9b2b20218538