Auto merge of #18550 - nnethercote:bug-1399758-take-2, r=heycam

Measure ImageValue objects (take 2).

We have about 11,500 of these when loading gmail in a Stylo-enabled build, from
SpecifiedUrls; the objects themselves account for about 1.3 MiB of memory, and
the strings within them about 2.9 MiB.

We also have a very small number of them on the Gecko side.

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