DevTools test harness: add InspectorTest.dumpProtocolMessages method.

upstream patch https://codereview.chromium.org/288663003/

BUG=

Review URL: https://codereview.chromium.org/287883002

git-svn-id: svn://svn.chromium.org/blink/trunk@174787 bbb929c8-8fbe-4397-9dbb-9b2b20218538