[DevTools] Make inspector-protocol tests work with messages passed as objects.

This mirrors the real inspector frontend logic,
where we already get objects instead of strings.

BUG=none

Review URL: https://codereview.chromium.org/444883004

git-svn-id: svn://svn.chromium.org/blink/trunk@179619 bbb929c8-8fbe-4397-9dbb-9b2b20218538