Port dev tools WebUI from URLFetcher to SimpleURLLoader

This needed to be done at some point. The motivation here was to fix an
extensions WebRequest API test with the Network Service enabled.

This also plumbs through a signal so extensions API test JS code can
detect whether Network Service is enabled. This is used to support
conditional expectations in WebRequest tests since the test-only
event behavior may differ slightly between Network Service and old
networking.

Bug: 721414
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I009061f038eb3171721fe7645444695a77f3cfd3
Reviewed-on: https://chromium-review.googlesource.com/940447
Commit-Queue: Ken Rockot <[email protected]>
Reviewed-by: John Abd-El-Malek <[email protected]>
Reviewed-by: Matt Menke <[email protected]>
Reviewed-by: Pavel Feldman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#540183}
8 files changed