Whitelist WebFeatures DownloadInSandbox

To get a sense of which sites are having sandboxed download, so we might
be able to provide evidence that the blocking is worthwhile.

Bug: 539938
Change-Id: I672077892064b0f156a78aac1ff5be044f6c3ef8
Reviewed-on: https://chromium-review.googlesource.com/c/1356890Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613305}

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

@@ -76,6 +76,10 @@ bool IsAllowedUkmFeature(blink::mojom::WebFeature feature) {
           WebFeature::kSignedExchangeInnerResponseInMainFrame,
           WebFeature::kSignedExchangeInnerResponseInSubFrame,
           WebFeature::kWebShareShare,
+          WebFeature::kHTMLAnchorElementDownloadInSandboxWithUserGesture,
+          WebFeature::kHTMLAnchorElementDownloadInSandboxWithoutUserGesture,
+          WebFeature::kNavigationDownloadInSandboxWithUserGesture,
+          WebFeature::kNavigationDownloadInSandboxWithoutUserGesture,
       }));
   return opt_in_features->count(feature);
 }