Select precache resources based on field trial experiment group

components/precache/core/proto/precache.proto

Index: components/precache/core/proto/precache.proto
diff --git a/components/precache/core/proto/precache.proto b/components/precache/core/proto/precache.proto
index bfd4c508780911bc4cd8a2d0485674d2e022e04e..9bdd7af9b5cf1e9d8afef82f1a4c157328d5e089 100644
--- a/components/precache/core/proto/precache.proto
+++ b/components/precache/core/proto/precache.proto
@@ -20,8 +20,30 @@ 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