Auto merge of #17123 - KuoE0:bug-1358993, r=heycam

Store the pointer of reusable sheets in StylesheetLoader.

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To make stylo support reusable style sheets, we need to store the pointer of reusable style sheets we got from gecko. And we pass them back to gecko to reuse.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1358993](https://bugzilla.mozilla.org/show_bug.cgi?id=1358993).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because the test cases are in Gecko.

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