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More than once folks did not realize that the benchmark requires
building maps_go_webapk. Explain it in the error message.

Also explain more verbosely the very unobvious flags that are required
to make Telemetry happy.

Bug: 961624
Change-Id: Ibc2e895424c902cd7180a2e830eea60f3952f90e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605415
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658542}