Select precache resources based on field trial experiment group

components/precache/core/proto/precache.proto

 // Copyright 2013 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.
 
 syntax = "proto2";
 
 package precache;
 
 // Chrome requires this.
 option optimize_for = LITE_RUNTIME;
 
 // Information about a cacheable resource to be precached.
 message PrecacheResource {
   // The URL of the resource. This field must always be present.
   optional string url = 1;
 };
 
 // A manifest of cacheable resources to be precached for a specific host.
 message PrecacheManifest {
   // List of resources that we predict that the user will need if they are
   // likely to fetch the host.
   repeated PrecacheResource resource = 1;
+
+  // Experiments running on this manifest.
+  optional PrecacheExperiments experiments = 2;
 };
 
+message PrecacheExperiments {
+  // A mapping between experiment groups and the resources that should be
+  // considered for the experiment.
+  map<fixed32, PrecacheResourceSelection> resources_by_experiment_group = 1;
+}
+
+message PrecacheResourceSelection {
+  // Select the resources as a std::bitset over the resources listed in the
+  // manifest.
+  //
+  // The bitset should be loaded as:
+  //   std::bitset selection(resource_selection.bitset());
+  // Then manifest.resource(i) is selected for the experiment iff
+  // selection.test(i).
+  //
+  // A missing bitset means that the experiment applies to all the resources.
+  optional fixed64 bitset = 1 [default = 0xFFFFFFFFFFFFFFFF];
+}
+
 message PrecacheConfigurationSettings {
   // The maximum rank of the user's most visited hosts to consider precaching
   // resources for, starting from 1. For example, a value of 10 means that only
   // hosts that are in the user's top 10 most visited hosts will be considered
   // as starting URLs for resource precaching. This is specified by the server
   // for testing purposes, so that it's easy to adjust how aggressively
   // resources are precached.
   // Values that are zero or lower indicate that none of the user's top sites
   // will be used for precaching.
   optional int64 top_sites_count = 1 [default = 100];
@@ skipping 11 matching lines @@
   // resources larger than this will be cancelled. This max applies only to new
   // downloads; cached resources are not capped.
   optional uint64 max_bytes_per_resource = 4 [default = 500000 /* 500 KB */];
 
   // The maximum number of bytes per precache run. While precaching, the total
   // number of bytes used for resources is tallied -- this includes new
   // downloads as well as cached resources. After this limit is reached, no
   // other resources will be downloaded.
   optional uint64 max_bytes_total = 5 [default = 10000000 /* 10 MB */];
 };