[AdTracking] Speed up SubresourceFilter::IsAdResource

Check if the frame is an ad frame before checking the filter list.

Bug: 807640
Change-Id: Ic8a3e1e4ceca3ed64ff39895c697205d3c1477b0
Reviewed-on: https://chromium-review.googlesource.com/1018426
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552144}

third_party/blink/renderer/core/loader/subresource_filter.cc

@@ -98,6 +98,9 @@ bool SubresourceFilter::GetIsAssociatedWithAdSubframe() {
 bool SubresourceFilter::IsAdResource(
     const KURL& resource_url,
     WebURLRequest::RequestContext request_context) {
+  if (subresource_filter_->GetIsAssociatedWithAdSubframe())
+    return true;
+
   WebDocumentSubresourceFilter::LoadPolicy load_policy;
   if (last_resource_check_result_.first ==
       std::make_pair(resource_url, request_context)) {
@@ -107,10 +110,7 @@ bool SubresourceFilter::IsAdResource(
         subresource_filter_->GetLoadPolicy(resource_url, request_context);
   }
 
-  // If the subresource cannot be identified as an ad via load_policy, check if
-  // its frame is identified as an ad.
-  return load_policy != WebDocumentSubresourceFilter::kAllow ||
-         subresource_filter_->GetIsAssociatedWithAdSubframe();
+  return load_policy != WebDocumentSubresourceFilter::kAllow;
 }
 
 void SubresourceFilter::ReportLoad(