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..20410f971ae285e14a52b19c1f9e0089a713de61 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 kPerfMeasurementRateParameterName[];

[engedy, 2016/12/12 15:32:36]

nit: Abbreviations are discouraged as per the style guide unless they are very
common. Please s/Perf/Performance/g, it's only a few more characters.

[pkalinnikov, 2016/12/12 18:47:29]

Done.

+
 // 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 +48,12 @@ ActivationScope GetCurrentActivationScope();
 // variation param is empty, returns most conservative ActivationList::NONE.
 ActivationList GetCurrentActivationList();
 
+// Returns current performance measurement rate for the SubresourceFilter

[engedy, 2016/12/12 15:32:36]

Slightly more succinct phrasing suggestion:

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.

[pkalinnikov, 2016/12/12 18:47:29]

Done.

+// component. This parameter instructs SubresourceFilter to measure performance
+// for roughly |rate| * N page loads out of N. Returned value varies from 0 to
+// 1.0, and equals 1 if the variation param is empty of malformed.
+double GetCurrentPerfMeasurementRate();

[engedy, 2016/12/12 15:32:36]

Drop 'Current' for simplicity.

[pkalinnikov, 2016/12/12 18:47:29]

Done.

+
 }  // namespace subresource_filter
 
 #endif  // COMPONENTS_SUBRESOURCE_FILTER_SUBRESOURCE_FILTER_FEATURES_H_