pgo-builder: adds ability to specify weights per benchmark

This allows passing in a weights dictionary that maps from
benchmark to weight. The weight is used when merging profiles.

Bug: 363195532
Change-Id: Ib752bf0aed769e23cb503973740c249a48065502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5825299


Reviewed-by: Zhaoyang Li <zhaoyangli@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349410}