Make Blob a Compat type

[schmidt-sebastian]

This is mostly a no-op but it aligns the implementation of Blob with the rest of the API types

[changeset-bot]

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Latest commit: af08d0a

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[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (d934f74) and head commit (81f15d4).

Test Logs

• Base (d934f74d): https://github.com/firebase/firebase-js-sdk/actions/runs/370810394
• Head (81f15d41): https://github.com/firebase/firebase-js-sdk/actions/runs/371280150