[size-adjust] Implement @font-face descriptor 'size-adjust'

'size-adjust' is specified as a scaling factor to 'font-size' before
font selection, similar to 'font-size-adjust' property. Therefore, this
patch implements it in a way similar to 'font-size-adjust' that, both
affect the result of FontDescription::EffectiveFontSize().

Spec: https://drafts.csswg.org/css-fonts-5/#descdef-font-face-size-adjust

Bug: 1137633
Change-Id: I157705bd1a9c1e58ec3c4f41bbff7a37d1f7e3dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2773716
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866291}