Introduce performance measurement varitation parameter for SubresourceFilter.

components/subresource_filter/core/browser/subresource_filter_features.cc

Index: components/subresource_filter/core/browser/subresource_filter_features.cc
diff --git a/components/subresource_filter/core/browser/subresource_filter_features.cc b/components/subresource_filter/core/browser/subresource_filter_features.cc
index e85213c37a684ff52f9c6edb9865e3453b45eb8d..9ac0acf69922b6ce64ce4dcb5c8f75076d3bf7fe 100644
--- a/components/subresource_filter/core/browser/subresource_filter_features.cc
+++ b/components/subresource_filter/core/browser/subresource_filter_features.cc
@@ -6,6 +6,7 @@
 
 #include <string>
 
+#include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 #include "components/variations/variations_associated_data.h"
@@ -30,6 +31,9 @@ const char kActivationListSocialEngineeringAdsInterstitial[] =
     "social_engineering_ads_interstitial";
 const char kActivationListPhishingInterstitial[] = "phishing_interstitial";
 
+const char kPerformanceMeasurementRateParameterName[] =
+    "performance_measurement_rate";
+
 ActivationState GetMaximumActivationState() {
   std::string activation_state = variations::GetVariationParamValueByFeature(
       kSafeBrowsingSubresourceFilter, kActivationStateParameterName);
@@ -70,4 +74,13 @@ ActivationList GetCurrentActivationList() {
   return activation_list_type;
 }
 
+double GetPerformanceMeasurementRate() {
+  const std::string rate = variations::GetVariationParamValueByFeature(
+      kSafeBrowsingSubresourceFilter, kPerformanceMeasurementRateParameterName);
+  double value = 0;
+  if (!base::StringToDouble(rate, &value) || value < 0)
+    return 0;
+  return value < 1 ? value : 1;
+}
+
 }  // namespace subresource_filter