Change field_trials type to hashes in permission report proto

chrome/common/safe_browsing/permission_report.proto

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Protocol buffer for permission reports sent to the Safe Browsing client-side
 // detection (CSD) frontends. This should stay in sync with the Safe Browsing 
 // server-side protocol buffer.
 
 syntax = "proto2";
 
@@ skipping 16 matching lines @@
   // When the request was initiated.
   repeated RequestTrigger request_trigger = 4;
 
   // The action the user took. Required.
   optional Action action = 5;
 
   // The UI used to complete the action.
   optional SourceUI source_ui = 6;
 
   // The relevant field trials enabled for this report.
-  // TODO(stefanocs): Update field_trials field to store hashes instead of
-  // string.
-  repeated string field_trials = 7;
+  repeated FieldTrial field_trials = 7;
 
   // Platform
   enum PlatformType {
     PLATFORM_TYPE_UNSPECIFIED = 0;
     DESKTOP_PLATFORM = 1;
     ANDROID_PLATFORM = 2;
   }
 
   // Various request triggers we want to record, e.g. on page load.
   enum RequestTrigger {
@@ skipping 32 matching lines @@
     MIDI_SYSEX = 1;
     PUSH_MESSAGING = 2;
     NOTIFICATIONS = 3;
     GEOLOCATION = 4;
     PROTECTED_MEDIA_IDENTIFIER = 5;
     MIDI = 6;
     DURABLE_STORAGE = 7;
     AUDIO_CAPTURE = 8;
     VIDEO_CAPTURE = 9;
   }
+
+  // Description of a field trial or experiment that the user is currently
+  // enrolled in. All metrics reported in this upload can potentially be
+  // influenced by the field trial.
+  message FieldTrial {
+    // The name of the field trial, as a 32-bit identifier. Currently, the
+    // identifier is a hash of the field trial's name.
+    optional fixed32 name_id = 1;
+
+    // The user's group within the field trial, as a 32-bit identifier.
+    // Currently, the identifier is a hash of the group's name.
+    optional fixed32 group_id = 2;
+  }
 }