Introduce constants for histogram names in tests.

components/subresource_filter/content/renderer/subresource_filter_agent.cc

Index: components/subresource_filter/content/renderer/subresource_filter_agent.cc
diff --git a/components/subresource_filter/content/renderer/subresource_filter_agent.cc b/components/subresource_filter/content/renderer/subresource_filter_agent.cc
index 90c1230bd6ae917a715de1d708ee47e552083271..3eb3a210183613ad7e5db6b7af6a4a601af1fc1d 100644
--- a/components/subresource_filter/content/renderer/subresource_filter_agent.cc
+++ b/components/subresource_filter/content/renderer/subresource_filter_agent.cc
@@ -80,6 +80,7 @@ void SubresourceFilterAgent::RecordHistogramsOnLoadCommitted() {
 void SubresourceFilterAgent::RecordHistogramsOnLoadFinished() {
   DCHECK(filter_for_last_committed_load_);
   const auto& statistics = filter_for_last_committed_load_->statistics();
+
   UMA_HISTOGRAM_COUNTS_1000(
       "SubresourceFilter.DocumentLoad.NumSubresourceLoads.Total",
       statistics.num_loads_total);