Allow exporting the current selection as CSV.

No-try: true
Bug: v8:7266
Change-Id: Idd275e749506d2a195a132efa5ec08ebb21ca72f
Reviewed-on: https://chromium-review.googlesource.com/870781Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50668}