RenderText::computeCanUseSimplifiedTextMeasuring() has:

    TextRun run(m_text);
    run.setCharacterScanForCodePath(false);
    if (font.codePath(run) != FontCascade::Simple)
        return false;

and FontCascade::codePath() has:

    if (!run.characterScanForCodePath())
        return Simple;

and

bool characterScanForCodePath() const { return m_characterScanForCodePath; }

this code sure looks like:
1. Explicitly set m_characterScanForCodePath to false
2. If m_characterScanForCodePath is false, use the simple text codepath