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Extension profile extender for profile generator and a new benchmark for
startup time on a profile that loads extensions. Also for this purpose, a
script to maintain a static set of extensions in cloud storage, and small
additions to python gsutil wrapper (cloud_storage).

BUG=444230
CQ_EXTRA_TRYBOTS=tryserver.chromium.perf:linux_perf_bisect;tryserver.chromium.perf:mac_perf_bisect

Review URL: https://codereview.chromium.org/1240703003

Cr-Commit-Position: refs/heads/master@{#339807}