Tracking issue: reduce diff with upstream

[nicoburns]

We want to upstream as many changes as possible.

Current commits containing diffs on top of upstream

• Commit our changes on top of upstream Stylo
• Fixup for https://phabricator.services.mozilla.com/D226265
• Enable prefers-color-scheme
• Remove Servo-specific code from malloc_size_of
• Remove dependency on webrender_api

(ticked commits have been upstreamed since the more recent sync)

Upstreamed code

• Remove dependency on webrender_api #92
• Remove Servo-specific code from malloc_size_of #91
• servo: Add support for parsing :is() and :where() #80

Changes that align downstream with upstream:

• Delete size_of_test crate #87
• Remove dependency on time #68
• Eliminate thread pool mutex #97

[Loirooriol]

Fixup for https://phabricator.services.mozilla.com/D226265

This one was aligning downstream with upstream.