Bug 157846. Part 0: Make tests more robust to padding changes and clipping of overflowing glyph edges. r=mats

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extra : rebase_source : f7d8ecb0df3fbbea593e46e515db47ee705392c7
Bug 157846. Part 0.1: When reflowing a non-rootframe reflow root, preserve its used padding in case that differs from its CSS computed padding for some reason. r=mats

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extra : rebase_source : 15631dc5a5f40dc2d95b1eaa7698fba635763626
Bug 157846. Part 0.2: Relax heuristic used to detect 'monospace' font for <input> sizing so that it at least includes 'monospace' font-family in Windows 7. r=mats

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extra : rebase_source : 70bfb8df0c582433ea786b7092536806186ca3d3