[blinkpy] Register mac12 port (Monterey) and add new baselines

* The 'mac' fallback path has been repurposed for 'mac12'. 'mac11'-only
  baselines will go in 'platform/mac11'.
* Fix the reliability of the Mac version retrieved from the platform.
* Add 'Mac12*' to '*Expectations' headers.

Bug: 1323327
Change-Id: I8f600f798a1bca48d799224bbfcaf95f5f4fdaac
Cq-Include-Trybots: luci.chromium.try:mac10.13-blink-rel,mac10.14-blink-rel,mac10.15-blink-rel,mac11.0-blink-rel,mac11.0.arm64-blink-rel,mac12.0-blink-rel,mac12.0.arm64-blink-rel,win7-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3648124
Reviewed-by: Weizhong Xia <[email protected]>
Commit-Queue: Jonathan Lee <[email protected]>
Reviewed-by: Erik Staab <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1007391}
diff --git a/BUILD.gn b/BUILD.gn
index e472f3b..5381b33 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1415,7 +1415,9 @@
         "//third_party/blink/web_tests/platform/mac-mac10.13/",
         "//third_party/blink/web_tests/platform/mac-mac10.14/",
         "//third_party/blink/web_tests/platform/mac-mac10.15/",
+        "//third_party/blink/web_tests/platform/mac-mac11/",
         "//third_party/blink/web_tests/platform/mac-mac11-arm64/",
+        "//third_party/blink/web_tests/platform/mac-mac12-arm64/",
       ]
     }