Network Service: Update URLLoader::OpenFilesForUpload to open limited number of files at a time

This CL updates URLLoader::OpenFilesForUpload to open limited number of
files at a time.

Bug: 939606, 944218
Change-Id: I69f262e122fbdd9216393656d75645cd5b60f450
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1520317
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Commit-Queue: Jun Cai <juncai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644509}
2 files changed