sharing: harden ShareRanking code

In particular:
* Many DCHECKs are promoted into DCHECKs
* Indexes and lengths are represented with size_t instead of unsigned
  int
* (begin, end) std::vector constructions are replaced with uses of
  base::ToVector(span)

There is at least one memory corruption bug lurking in this code (the
linked one) so hopefully this will help flush it out.

Bug: 331577262
Change-Id: I9266f2a134964fc7b7741289af991a69ec79f4cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5698134
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1328321}