Use SetHeader instead of AddHeadersFromString

This CL refactors instances of `net::HttpRequestHeaders::AddHeadersFromString`
that use compile-time constant string literals to instead use individual `SetHeader` calls.

This improves readability and avoids the overhead of parsing the header string at runtime.

Bug: 41400723


Change-Id: Id4a182f44220bf16cc9fedb90ba5d8343d969ed3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6807764
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Doug Turner <dft@google.com>
Cr-Commit-Position: refs/heads/main@{#1495467}