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This adds a smallish intro to what service workers are before
diving into the implementation details.

Bug: 795989
Change-Id: I00155d1c7c987e3382e69ba67b4aa8b6de077aa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981243
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727394}