style: Add a FIXME comment that I need to write a test-case for.

[emilio]

This change is 

[highfive]

Heads up! This PR modifies the following files:

• @bholley: components/style/stylist.rs
• @canaltinova: components/style/stylist.rs

[highfive]

Warning

• These commits modify style code, but no tests are modified. Please consider adding a test!

[emilio]

@bors-servo r+

[bors-servo]

📌 Commit 0e78138 has been approved by emilio

[bors-servo]

⌛ Testing commit 0e78138 with merge fab8042...

[bors-servo]

💔 Test failed - linux-rel-wpt

[CYBAI]

Intermittent with

{"status": "TIMEOUT", "group": "default", "message": null, "stack": null, "subtest": null, "test": "/css/css-backgrounds/box-shadow-syntax-001.xht", "line": 28470, "action": "test_result", "expected": "FAIL"}
{"status": "PASS", "group": "default", "message": null, "stack": null, "subtest": null, "test": "/_mozilla/css/border_radius_elliptical_a.html", "line": 40711, "action": "test_result", "expected": "FAIL"}

[jdm]

@bors-servo: retry

• #19597

[bors-servo]

⚡ Previous build results for android, arm32, arm64, linux-dev, linux-rel-css, mac-dev-unit, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, windows-msvc-dev are reusable. Rebuilding only linux-rel-wpt...

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel-css, linux-rel-wpt, mac-dev-unit, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, windows-msvc-dev
Approved by: emilio
Pushing fab8042 to master...