DevTools: Add a callback param to expandConsoleMessages method.

The InspectorTest.expandConsoleMessages() may be async, so having a callback parameter will be a good hint.

BUG=
R=vsevik

Review URL: https://codereview.chromium.org/26977004

git-svn-id: svn://svn.chromium.org/blink/trunk@159459 bbb929c8-8fbe-4397-9dbb-9b2b20218538