Share specified URLs with Gecko
This makes use of our Arc offset machinery to allow Gecko to store Arc<String> values and ensure there are used appropriately.
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- [x] These changes fix [bug 1397971](https://bugzilla.mozilla.org/show_bug.cgi?id=1397971).
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Measure memory usage of Stylo's Rule Tree.
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This is for https://bugzilla.mozilla.org/show_bug.cgi?id=1394729, which was r=heycam.
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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.
We fail bindgen layout tests on 32 bit because FontComputationData does not accurately reflect the size/alignment of the rust-side replacement.
We move it to the end so that alignment is more straightforward, and
use a NonZero enum so that the representation can be more accurately
handled on the C++ side.
MozReview-Commit-ID: 8GlSma3Wl9W
This patch also removes the duplication of style contexts during the
restyle, because otherwise pointer equality of ServoComputedValues stops
holding (and we assert on that in a few places)
