DevTools: Add FakeExecutionContextList class in runtimeModel

So we will add it in worker's runtimeModel later


BUG=

Review URL: https://codereview.chromium.org/201623005

git-svn-id: svn://svn.chromium.org/blink/trunk@169359 bbb929c8-8fbe-4397-9dbb-9b2b20218538