Record UKM for kHiddenUntilFoundAttribute

I have verified that the usage is less than 1%
See the linked bug for screenshots

(cherry picked from commit 0b7d37c)

Bug: 1449399
Change-Id: Ic13600b71af50c54abb1cdc4ea98beeefb0e9613
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4571574
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1150268}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4575501
Reviewed-by: Krishna Govind <govind@chromium.org>
Owners-Override: Krishna Govind <govind@chromium.org>
Commit-Queue: Krishna Govind <govind@chromium.org>
Cr-Commit-Position: refs/branch-heads/5790@{#146}
Cr-Branched-From: 1d71a33-refs/heads/main@{#1148114}

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

@@ -292,6 +292,7 @@ UseCounterMetricsRecorder::GetAllowedUkmFeatures() {
            WebFeature::
                kServiceWorkerBypassFetchHandlerForMainResourceByOriginTrial,
            WebFeature::kCascadedCSSZoomNotEqualToOne,
-           WebFeature::kV8Window_QueryLocalFonts_Method}));
+           WebFeature::kV8Window_QueryLocalFonts_Method,
+           WebFeature::kHiddenUntilFoundAttribute}));
   return *opt_in_features;
 }