Update README of performance.measureMemory tests

Bug: 1085129
Change-Id: I16ee23ac320300b2e408aca4b8a4873b1f1594ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624674
Auto-Submit: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842621}

third_party/blink/web_tests/external/wpt/measure-memory/README.md

-# Tentative tests for performance.measureMemory API
+# Tests for performance.measureMemory API
 
-Tests in this directory are for the proposed performance.measureMemory API.
-This is not yet standardised and browsers should not be expected to pass
-these tests.
+Tests in this directory are for the performance.measureMemory API:
 
-See the explainer at https://github.com/WICG/performance-measure-memory
-for more information about the API.
+- Specification: https://wicg.github.io/performance-measure-memory
+- Explainer: https://github.com/WICG/performance-measure-memory