Refactor inspector-protocol/layers tests for better extensibility

- support initialize_ functions in same manner we do for front-end test suite;
- extract common layer test code to layer-protocol-test.js
- get rid of step(), use InspectorTest.sendCommand() + InspectorTest.wrapCallback()

Review URL: https://codereview.chromium.org/110353002

git-svn-id: svn://svn.chromium.org/blink/trunk@163443 bbb929c8-8fbe-4397-9dbb-9b2b20218538