Add three WCv0 reverse origin trial counters to UKM

While the usage of these three features (particularly
kHTMLImportsOnReverseOriginTrials) is over the 1% guideline,
they are (hopefully) trending toward zero. These three UKMs
will allow us to track site usage of these three features
more easily, to drive down usage.

Bug: 946875
Change-Id: I9b93714c7030a6f00c909b2eaa1db44a53471a6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1791627
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695157}

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

@@ -49,8 +49,11 @@ UseCounterPageLoadMetricsObserver::GetAllowedUkmFeatures() {
           WebFeature::kPaymentRequestShowWithoutGesture,
           WebFeature::kHTMLImports,
           WebFeature::kHTMLImportsHasStyleSheets,
+          WebFeature::kHTMLImportsOnReverseOriginTrials,
           WebFeature::kElementCreateShadowRoot,
+          WebFeature::kElementCreateShadowRootOnReverseOriginTrials,
           WebFeature::kDocumentRegisterElement,
+          WebFeature::kDocumentRegisterElementOnReverseOriginTrials,
           WebFeature::kCredentialManagerCreatePublicKeyCredential,
           WebFeature::kCredentialManagerGetPublicKeyCredential,
           WebFeature::kCredentialManagerMakePublicKeyCredentialSuccess,