Convert HttpTransaction to ByteSize

Throughout the network stack many layers pass byte counts around as
signed ints. It's often unclear which of those ints can hold negative
values in practice (such as -1 to indicate errors).

This makes it hard to deprecate ByteCount (signed) because the
replacement, ByteSize, is unsigned, and high-level ByteCount fields are
populated with values that come from deep in the network stack (eg. at
https://crsrc.org/c/third_party/blink/renderer/platform/exported/resource_load_info_notifier_wrapper.cc;l=201;drc=64820206068b8feaf4b9d4de22362cfcf047b8b6).

This replaces HttpTransaction methods returning int64_t, and all
implementations, with ByteSize. Followups can bubble up the ByteSize
type through the rest of the stack, checking whether negatives need to
be handled at each layer.

ByteSize holds values in the range [0,
std::numeric_limits<int64_t>::max()], so the call sites that convert
back to int64_t are guaranteed not to overflow.

All implementations already use unsigned data sources except
HttpNetworkTransaction which reads an int, but already checks for
positive values.

Internally, ByteSize CHECK's on overflow, so the only semantic change in
this patch is a safety improvement: previously out-of-range values could
overflow int64_t, now they will CHECK if that happens.

Bug: 448661443
Change-Id: Ic3d2df42dffdeddf5ff69e17fd3149fbbcafed62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7726691
Auto-Submit: Joe Mason <joenotcharles@google.com>
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1610804}