This patch adds measurement of Selectors within StyleRule. This requires
exposing the pointer within ThinArc.
The patch also adds measurement of the several CssRule variants, in order to
measure nested CssRules (and PropertyDeclarationBlocks) within them:
DocumentRule, MediaRule, PageRule, SupportsRule.
The patch provides FFI access to Gecko's SeenPtrs type from Rust, in
order to record what has already been measured when measuring Arcs. (The
SeenPtrs must be initialized on the Gecko side because the same table is
reused for measuring all Elements within a window, because Elements can
share ComputedValues.) I have confirmed with DMD that this is working
correctly.
The patch also introduces MallocSizeOfRepeats, which is like
MallocSizeOf but takes a SizeOfState, which holds a SeenPtrs table.
This reverts commit ebfc8f5858, reversing
changes made to 5585ff2c44.
Animation code can reenter and create a new TLS context from the traversal
SequentialTask, so this won't work as written.
