Domain Reliability: Add sample_rate field to Beacon

components/domain_reliability/beacon.h

Index: components/domain_reliability/beacon.h
diff --git a/components/domain_reliability/beacon.h b/components/domain_reliability/beacon.h
index 0822c742761747bf00347aef5ecf279b44f35633..1cb88e19e73395af2ec0ca716631d5488510499f 100644
--- a/components/domain_reliability/beacon.h
+++ b/components/domain_reliability/beacon.h
@@ -68,6 +68,8 @@ struct DOMAIN_RELIABILITY_EXPORT DomainReliabilityBeacon {
   // caused by an upload that itself contained no beacons caused by uploads,
   // et cetera.
   int upload_depth;
+  // The probability that this request had of being reported ("sample rate").
+  double sample_rate;
 
   // Okay to copy and assign.
 };