Control data saver pingback reporting fraction with protobuf

components/data_reduction_proxy/proto/client_config.proto

Index: components/data_reduction_proxy/proto/client_config.proto
diff --git a/components/data_reduction_proxy/proto/client_config.proto b/components/data_reduction_proxy/proto/client_config.proto
index 7729bcdb4a7f37242f9ca1b74ac5a0d25e87c463..98cc093e0d4b565d1ce9fd9d9a305ab70c243761 100644
--- a/components/data_reduction_proxy/proto/client_config.proto
+++ b/components/data_reduction_proxy/proto/client_config.proto
@@ -18,20 +18,28 @@ message ClientConfig {
   // for some time thereafter.
   optional Timestamp refresh_time = 2;
   // The proxy configuration the client should use to connect to the Data Saver
   // service.
   optional ProxyConfig proxy_config = 3;
   // The duration after which the client should request a new configuration. The
   // session_key is guaranteed to be valid through this time and may be valid
   // for some time thereafter. If both refresh_duration and refresh_time are
   // present, refresh_duration should take priority.
   optional Duration refresh_duration = 4;
+  // Configuration information for reporting pageload metrics.
+  optional PageloadMetricsConfig pageload_metrics_config = 5;
+}
+
+// The configuration for reporting pageload metrics.
+message PageloadMetricsConfig {
+  // The fraction of pageloads for which to report pageload metrics.
+  optional float reporting_fraction = 1;
 }
 
 // N.B.:
 // The configuration service that sends the ClientConfig uses Timestamp and
 // Duration to conform to Google API standards. These proto messages should
 // live in a shared location in the Chromium tree, but for now we duplicate
 // them here.
 
 // A Timestamp represents a point in time independent of any time zone
 // or calendar, represented as seconds and fractions of seconds at