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This change adds tests for measuring the performance of open, add,
and delete APIs of Cache Storage. Also adds support for running tests
in workers.

Design doc: https://docs.google.com/document/d/1bGBWHAsIsrQ0hrjU7oEVAcGSBS9gFSPcDL9nVWXDeeE/edit

Bug: 1019097
Change-Id: Ib173c699cda3ae9984ff20500e81b666b49d4e35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935928
Commit-Queue: Ting Shao <ting.shao@intel.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729667}