Throttle script request incidents to one per user.

BUG=461979
TEST=NONE

Review URL: https://codereview.chromium.org/961483003

Cr-Commit-Position: refs/heads/master@{#318192}

chrome/browser/safe_browsing/incident_reporting/script_request_incident.cc

@@ -26,11 +26,14 @@ IncidentType ScriptRequestIncident::GetType() const {
 }
 
 std::string ScriptRequestIncident::GetKey() const {
-  return payload()->script_request().script_digest();
+  // Use a static key in addition to a fixed digest below to ensure that only
+  // one incident per user is reported.
+  return "script_request_incident";
 }
 
 uint32_t ScriptRequestIncident::ComputeDigest() const {
-  // Return a constant to ensure that only one incident per digest is reported.
+  // Return a constant in addition to a fixed key above to ensure that only one
+  // incident per user is reported.
   return 42;
 }