Remove a raw pointer from NavigationURLLoaderImpl::ParseHeaders

To simplify lifetime management of `network::mojom::URLResponseHead`.

Previously, the `network::mojom::URLResponseHead` is kept alive as a
bound parameber of `continuation` callback and a raw pointer is passed
to `ParseHeaders()`.

After this CL, `network::mojom::URLResponseHeadPtr` is directly passed
through `ParseHeaders()` and to `continuation`.

Bug: 434182226
Change-Id: Ie12098430769c321519dc9e97392bad1b4560ce2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6787965
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494341}