service worker: Don't record fetch timing UMAs when DevTools is attached

When DevTools is attached, some instrumentation code is executed and
these could cause noise.

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Bug: 877972
Change-Id: I7c45739ef7467952b3501413044323873181eb65
Reviewed-on: https://chromium-review.googlesource.com/c/1263535
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597805}
2 files changed