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This CL adds a method to the Network domain that DevTools can use to
fetch resources in a more secure way. Such resources are, for example,
source maps, or files referenced by source maps. The improvement over
the old implementation is that the fetch now is very similar to a fetch
in the page and uses the correct settings for SiteForCookies and NIK.

Initially, we had planed to change source map fetching to CORS, but this
would be a breaking change, so due to COVID-19 and the hold on breaking
changes, this CL uses a no-CORS fetch with CORB blocking disabled to
fetch source maps. CORB-blocking must be disabled because source maps
may be valid JSON.

The front-end can provide a frame_id (in DevTools terms, a devtools
frame token in Chromium terms) and the page will look for this frame
id in its sub resources. The fetch will then occur using a loader that
is very similar to the loaders for that frame.

Change-Id: Ideb36adbd79b9a36e6d3333299dcd036eb7a1332
Bug: chromium:1069378
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027416
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810177}