… rulethickness"

This was reverted because the frac-1 test is failing on all windows platform
not just Win10.

Original change's description:

> Revert "[mathml] Use post.underlineThickness for the default fraction rulethickness"
>
> This reverts commit d01e107517ff5b1da0544be36c5caaf30c27456e.
>
> Reason for revert: <breaks win7 bots - https://ci.chromium.org/p/chromium/builders/ci/Win7%20Tests%20%281%29>
>
> Original change's description:
> > [mathml] Use post.underlineThickness for the default fraction rulethickness
> >
> > MathML Core currently defines fallback values to use when no MATH table
> > is available. This CL performs the remaining work to ensure fractions
> > align with the specification. It also adds WPT test to check these
> > fraction fallback parameters, although testing them extensively is
> > difficult.
> >
> > Bug: 6606, 1058369
> > Change-Id: Ic916c0c4b925674d526d78989988e57d8735bf4e
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410384
> > Commit-Queue: Frédéric Wang <fwang@igalia.com>
> > Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#807083}
>
> TBR=ikilpatrick@chromium.org,fwang@igalia.com,rbuis@igalia.com
>
> Change-Id: Ia8857539d69bbbafda2be5501ac1b766488be77d
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 6606
> Bug: 1058369
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412750
> Reviewed-by: David Bienvenu <davidbienvenu@google.com>
> Commit-Queue: David Bienvenu <davidbienvenu@google.com>
> Cr-Commit-Position: refs/heads/master@{#807144}

Bug: 6606, 1058369
Change-Id: I2fefb18b84c9604fcd40a013617a51aeadf56602
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414029
Reviewed-by: Rob Buis <rbuis@igalia.com>
Commit-Queue: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/master@{#807437}