Introduce performance measurement varitation parameter for SubresourceFilter.

components/subresource_filter/core/browser/subresource_filter_features.h

Index: components/subresource_filter/core/browser/subresource_filter_features.h
diff --git a/components/subresource_filter/core/browser/subresource_filter_features.h b/components/subresource_filter/core/browser/subresource_filter_features.h
index b19b2dc133359293dc9391dbaefc792709bc32b4..11f54ffab9ab426722b01095ef85016c6df62f07 100644
--- a/components/subresource_filter/core/browser/subresource_filter_features.h
+++ b/components/subresource_filter/core/browser/subresource_filter_features.h
@@ -30,6 +30,8 @@ extern const char kActivationListsParameterName[];
 extern const char kActivationListSocialEngineeringAdsInterstitial[];
 extern const char kActivationListPhishingInterstitial[];
 
+extern const char kPerformanceMeasurementRateParameterName[];
+
 // Returns the maximum degree to which subresource filtering should be activated
 // on any RenderFrame. This will be ActivationState::DISABLED unless the feature
 // is enabled and variation parameters prescribe a higher activation state.
@@ -46,6 +48,11 @@ ActivationScope GetCurrentActivationScope();
 // variation param is empty, returns most conservative ActivationList::NONE.
 ActivationList GetCurrentActivationList();
 
+// Returns a number in the range [0, 1], indicating the fraction of page loads
+// that should have extended performance measurements enabled. The rate will be
+// 0 unless a greater frequency is specified by variation parameters.
+double GetPerformanceMeasurementRate();
+
 }  // namespace subresource_filter
 
 #endif  // COMPONENTS_SUBRESOURCE_FILTER_SUBRESOURCE_FILTER_FEATURES_H_