Opt in HTML Imports to UKM USeCounter

Bug: 766694, 523952
Change-Id: Ic794281024e97783d9f2bde6f2ca6c7f2eefdbb8
Reviewed-on: https://chromium-review.googlesource.com/1027551Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553909}

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

@@ -27,6 +27,7 @@ bool IsAllowedUkmFeature(blink::mojom::WebFeature feature) {
           WebFeature::kUsbRequestDevice, WebFeature::kXMLHttpRequestSynchronous,
           WebFeature::kPaymentHandler,
           WebFeature::kPaymentRequestShowWithoutGesture,
+          WebFeature::kHTMLImports, WebFeature::kHTMLImportsHasStyleSheets,
       }));
   return opt_in_features.count(feature);
 }