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Makes it possible to --use-live-sites when invoking from command line.

Support for tools/perf/record_wpr was not tested, hence placing an
assert there to avoid surprises at a later stage. Documentation about
record_wpr [1] seems outdated as it does not mention wprgo, but I am not
sure how much outdated it is, could be only this tiny detail.

[1] Documentation page: "Telemetry: Record a Page Set"
    https://www.chromium.org/developers/telemetry/record_a_page_set

Bug: 760498
Change-Id: I2278eba06eeceb724c5ae6779409df00add90a4e
Reviewed-on: https://chromium-review.googlesource.com/1124857Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572527}