Fix Firestore build

[schmidt-sebastian]

This fixes an error in the JS build that has been showing up for me locally. It was introduced in #5808. Note that the error did not prevent the break the build - so that is a bit strange.

The main change here is that IndexedDbRemoteDocument cache goes back to the state before the PR and continues to use MutableDocuments.

[changeset-bot]

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Latest commit: 17ae09e

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[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (9295bb0) and head commit (0caecbf).

Test Logs

• Base (9295bb0b): https://github.com/firebase/firebase-js-sdk/actions/runs/1603106571
• Head (0caecbfb): https://github.com/firebase/firebase-js-sdk/actions/runs/1608604550

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (9295bb0) and head commit (0caecbf).

Test Logs

• Base (9295bb0): https://github.com/firebase/firebase-js-sdk/actions/runs/1603106571
• Head (0caecbf): https://github.com/firebase/firebase-js-sdk/actions/runs/1608604550