Add glue methods to get JSValue of a private reference

[CYBAI]

While working on servo/servo#27026, we've noticed calling GetScriptPrivate will hit a CRASH consistently in Windows. jdm noticed that this is caused by "can't return JSValue by value on Windows"; thus, we need to expose these 2 glue methods so that we can handle the GetScriptPrivate and GetModulePrivate correctly.

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This will help servo/servo#27026.

We might want to handle the ModulePrivate much more correctly in the future so it might be worth adding it here as well?

[CYBAI]

Btw, I just noticed the Microsoft documentation link in line 1072 is 404 now.

I tried to search it but looks like I can't find exactly same one.

• https://docs.microsoft.com/en-us/cpp/code-quality/annotating-function-parameters-and-return-values?view=vs-2019
• https://docs.microsoft.com/en-us/cpp/cpp/reference-type-function-returns?view=vs-2019

But found 2 links might be related; ☝️ is any of them I should update to the comment?

[jdm]

Please file an issue about the broken doc link.

[jdm]

@bors-servo r+

[bors-servo]

:pushpin: Commit cc7aeda has been approved by jdm

[bors-servo]

:hourglass: Testing commit cc7aeda0e37f7757960c2a29c7d249398233b5da with merge 716dede8811ed525d9aab1e44cea392609b35d0a...

[bors-servo]

:sunny: Test successful - checks-travis, status-appveyor Approved by: jdm Pushing 716dede8811ed525d9aab1e44cea392609b35d0a to master...