Cache and re-use pthread blob URL when using mainScriptUrlOrBlob

[sbc100]

Without this change the blobURL is constructed for each thread that is created.

Gemini seemed to think this might help with #26882

In any case this seems like obvious improvement, even though I imagine mainScriptUrlOrBlob has very few users.

[sbc100]

Hmm.. this didn't seems to fix the error, but probably worth landing anyway.

[sbc100]

@juj are you also seeing failure/flakes on your waterfall related to mainScriptUrlOrBlob?

[kripken]

Why is this an improvement? These blobs are not very large are they? (the referenced data might be, but this doesn't help there..?) Maybe I'm looking at this wrong.

[sbc100]

Maybe I'm misunderstanding. IIUC in this case the blob size is the entire size of the JS program (the one the runs in each pthread). This means that createObjectURL will create a string the size of the entire program, right? Maybe not huge, but not nothing.

[sbc100]

Anyway, it didn't fix the regression :(

[kripken]

I believe Blobs only refer to their data, though. They are thin wrappers. So this might reuse the wrapper but not save the large JS, since createObjectURL just refers to the Blob.

This is unlike a data URL where the actual data is copied.

[juj]

@juj are you also seeing failure/flakes on your waterfall related to mainScriptUrlOrBlob?

Commented on #26882 (comment)