Add StaleWhileRevalidateExperiment histograms.

net/BUILD.gn

Index: net/BUILD.gn
diff --git a/net/BUILD.gn b/net/BUILD.gn
index 7963a0a0cfb4b69422a84ae887b9766a5ec22132..dbd48f249f0963d15155ca4b6320db8e47b92322 100644
--- a/net/BUILD.gn
+++ b/net/BUILD.gn
@@ -1278,6 +1278,34 @@ source_set("simple_quic_tools") {
   ]
 }
 
+action_foreach("stale_while_revalidate_experiment_domains_dafsa") {
+  script = "//net/tools/dafsa/make_dafsa.py"
+  sources = [
+    "base/stale_while_revalidate_experiment_domains.gperf",
+  ]
+  outputs = [
+    "${target_gen_dir}/base/{{source_name_part}}-inc.cc",
+  ]
+  args = [
+    "{{source}}",
+    rebase_path("${target_gen_dir}/base/{{source_name_part}}-inc.cc",
+                root_build_dir),
+  ]
+}
+
+source_set("stale_while_revalidate_experiment_domains") {
+  sources = [
+    "base/stale_while_revalidate_experiment_domains.cc",
+    "base/stale_while_revalidate_experiment_domains.h",
+  ]
+  deps = [
+    ":net",
+    ":stale_while_revalidate_experiment_domains_dafsa",
+    "//base",
+  ]
+  configs += net_shared_configs
+}
+
 if (!is_ios) {
   executable("quic_client") {
     sources = [
@@ -1340,6 +1368,7 @@ test("net_unittests") {
     ":http_server",
     ":net",
     ":simple_quic_tools",
+    ":stale_while_revalidate_experiment_domains",
     ":test_support",
     "//base",
     "//base:i18n",