[rebaseline-cl] Do not fill in results for ports that skip tests

`rebaseline-cl` has a feature to substitute incomplete results with
those from other ports. However, intentionally skipped tests are
incorrectly treated as "missing", which causes `rebaseline-cl` to
download and leave behind baselines for those skipped ports (because the
optimizer ignores skipped tests).

Also, use `Port.operating_system()` instead of parsing the raw port name
to determine OS family (e.g., `mac-mac12-arm64` incorrectly parsed as OS
`mac-mac12` instead of `mac`).

This issue became more apparent after reducing tests run on mac10.15
(crrev.com/c/4967713).

Bug: 1481304
Test: git cl issue 5004848
Test: ./blink_tool.py rebaseline-cl --patchset=1 --no-trigger-jobs
Test: No extraneous mac10.15 or fuchsia baselines created.
Test: Results for `mac-mac12-arm64` substituted from another mac port.
Change-Id: Ibe68482bee849e9cc94b8b516eff8c1e102502c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5011277
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1221326}