Move WebUI HTTP request header byte range handling to a shared location

In a future commit, this code will be re-used in the implementation of
an in-renderer resource loader, which will not be located in
//content/browser.

Bug: 362511750
Change-Id: I1e93b762005756b61d503540537eba348f0430fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6033597
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Alex Yang <aycyang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1385830}