Base synthetic bold/italic decision only on @font-face properties

FontCustomPlatformData::GetFontPlatformData() should only override the
higher level's decision to synthetically make the font bold/italic if
it's a variable font and thus not needed, instead of inspecting the font
and ignoring the synthetic bold/italic request if the font is already
bold/italic.

Fixed: 409512705
Change-Id: I26f68bdc60130c476b2d68bf0b91158a979c0f0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6469892
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Max Ihlenfeldt <max@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1463996}