CookieStore: Add UKM use counters for CookieStoreAPI

This change adds CookieStoreAPI to UKM use counters to record usage
of the API and observe how it is being used to help with improvements
to the shape of the API.

CookieStoreAPI Explainer: https://github.com/WICG/cookie-store/blob/master/explainer.md

Change-Id: Ib27ee0d5c228eb8a3e67540a4f0a4bb8a513187d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149866Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760238}

components/page_load_metrics/browser/observers/use_counter/ukm_features.cc

@@ -106,6 +106,7 @@ UseCounterPageLoadMetricsObserver::GetAllowedUkmFeatures() {
           WebFeature::kTextFragmentAnchorMatchFound,
           WebFeature::kCookieNoSameSite,
           WebFeature::kCookieInsecureAndSameSiteNone,
+          WebFeature::kCookieStoreAPI,
           WebFeature::kDeviceOrientationSecureOrigin,
           WebFeature::kDeviceOrientationAbsoluteSecureOrigin,
           WebFeature::kDeviceMotionSecureOrigin,