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This add an window.testRunner API that sends do-nothing IPC message
roundtrip. This API can be used to build IPC benchmark using
JavaScript.

TEST=none (Will add blink-side LayoutTest once this is landed)
BUG=402185
R=darin@chromium.org, jam@chromim.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/478483005

Cr-Commit-Position: refs/heads/master@{#290063}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290063 0039d316-1c4b-4281-b951-d872f2087c98