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This enables the new integration test framework to support startup
tests that need to exercise activities in the inspected page prior to
starting the DevTools session.

This migrates console-format-startup.js as an example test.

Bug: 667560
Change-Id: Ifdd82135494ef7e05879297197e210a9b812d3ea
Reviewed-on: https://chromium-review.googlesource.com/803909
Commit-Queue: Will Chen <chenwilliam@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523549}