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In that mode isolate_driver.py will collect all arguments needed for
isolate.py invocation into *.isolated.gen.json file, but won't actually
run the isolation itself.

It's a feature of isolate_driver.py. Isolate client itself knows nothing about
this mode.

BUG=389227
R=maruel@chromium.org

Review URL: https://codereview.chromium.org/555553002

Cr-Commit-Position: refs/heads/master@{#293885}