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When the network service or NetS13nSW is enabled, requests
skip service worker loaders entirely when the controller has
no fetch event handler. This was broke assumptions in
DocumentThreadableLoader, since it expected service worker
machinery to return "fallback for CORS" responses for
certain requests when there was a controller.

Bug: 850839
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I32b058caacb14cba2e53dd8667fe9fc930a776e8
Reviewed-on: https://chromium-review.googlesource.com/1107435Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568796}