Bug 1561129 [wpt PR 17443] - KV Storage: update to match IDL, a=testonly

Automatic update from web-platform-tests
KV Storage: update to match IDL

Along with https://chromium-review.googlesource.com/c/chromium/src/+/1670572, this aligns the KV Storage implementation with WICG/kv-storage#68, which uses Web IDL to define the API. The observable changes are to:

* Enumerability of methods
* Adding toStringTag (affecting Object.prototype.toString.call)

This includes web platform tests that abuse the current idlharness.js infrastructure, plus some ad-hoc hand-written tests that we expect to be generated by future versions of idlharness.js once the relevant Web IDL pull requests are merged. It removes the existing API surface tests and helpers in favor of idlharness.js.

Bug: 931263
Change-Id: I9205d1a8b3040617cbb6200f825ba9ad250e61c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672137
Commit-Queue: Domenic Denicola <domenicchromium.org>
Reviewed-by: Joshua Bell <jsbellchromium.org>
Cr-Commit-Position: refs/heads/master{#671975}

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wpt-commits: 72da3e36e3657a358c4e74b495e582262b40a016
wpt-pr: 17443

UltraBlame original commit: e263779ccbe5a96dc55158b84f9a772ef548e200

testing/web-platform/tests/kv-storage/entries.https.html

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