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After crrev.com/c/1832202 renderer startup traces has been recording
correctly under the category "startup." Add the category to service
worker background tracing config so that we can collect these traces.

Bug: 561209
Change-Id: Ic27214be3f328dff47365083d8797dc7f36b3c46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1839278Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702769}