Introduce subresource_filter::ConfigurationList and make querying it cheap.

components/subresource_filter/core/browser/subresource_filter_features_test_support.cc

Index: components/subresource_filter/core/browser/subresource_filter_features_test_support.cc
diff --git a/components/subresource_filter/core/browser/subresource_filter_features_test_support.cc b/components/subresource_filter/core/browser/subresource_filter_features_test_support.cc
index 16b8354e0e7379f7a7215f2c88c0130604324050..5e43f17dd69cede72da0465e2a4ed30d97640408 100644
--- a/components/subresource_filter/core/browser/subresource_filter_features_test_support.cc
+++ b/components/subresource_filter/core/browser/subresource_filter_features_test_support.cc
@@ -65,10 +65,18 @@ ScopedSubresourceFilterFeatureToggle::ScopedSubresourceFilterFeatureToggle(
   }
 
   scoped_feature_list_.InitWithFeatureList(std::move(feature_list));
+
+  // Force the active ConfigurationList to be reparsed on next access so that
+  // the variation parameters come into effect.
+  ClearCachedActiveConfigurations();
 }
 
 ScopedSubresourceFilterFeatureToggle::~ScopedSubresourceFilterFeatureToggle() {
   variations::testing::ClearAllVariationParams();
+
+  // Force the active ConfigurationList to be reparsed on next access, so that
+  // the overrides from this instance are no longer in effect.
+  ClearCachedActiveConfigurations();
 }
 
 }  // namespace testing