[rebaseline-cl] Generate dummy all-pass baselines when necessary

The warning about clobbering all-pass statuses (crrev.com/c/4334979) is
not useful when the affected builder/test passes expectedly (so it
generates no `actual_text` to download). In this change, `rebaseline-cl`
instead automatically generates a dummy baseline that `run_web_tests.py`
and `optimize-baselines` recognize as all-pass. This preserves subtest
-level coverage for failing platforms.

These dummy baselines should rarely be necessary, so this change's
impact on checkout size should be negligible. The dummy baselines are
self-documenting and link to the relevant web test documentation, which
this change updates.

Bug: 1324638
Test: To test the `run_web_tests.py` side:
Test: For `external/wpt/badging/badge-success.https.html`, create an
Test: abbreviated all-pass baseline for linux (host platform) and a
Test: failing baseline for win.
Test: `./run_web_tests.py -t Default external/wpt/badging` succeeds
Change-Id: I4a94fb4fda5c566ab0b5838c8ac913c6ee6cccce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4385729
Reviewed-by: Weizhong Xia <weizhong@google.com>
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1124532}