Enable "gn check" for //third_party/blink/renderer/platform/network

It has a circular dependency with platform/loader:loader. This CL adds
allow_circular_includes_from to platform/loader:loader.

This CL has no behavior changes.

Bug: 800764
Change-Id: Ib511a3c8628e1e39b6c458cd51d0dd9a8598bd9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2400588


Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805624}