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This is the experimental IDBIndex.getAll implementation for
retrieving multiple values in a single request as proposed in
https://www.w3.org/2008/webapps/wiki/IndexedDatabaseFeatures

Mozilla's documentation for IDBIndex.getAll is at:
https://developer.mozilla.org/en-US/docs/Web/API/IDBIndex/getAll

In order to use run Chrome with: --enable-experimental-web-platform-features

BUG=457450

Review URL: https://codereview.chromium.org/1147433002

Cr-Commit-Position: refs/heads/master@{#330631}