[Sync] Implements the loopback sync server.

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index a9674b35270429661247873fa25dcd7b74417405..1c1e7a6fbc92a61ff94272d9d22949db6331bb29 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -584,6 +584,10 @@ static_library("sync") {
     ]
   }
 
+  if (is_win) {
+    deps += [ "//components/sync:loopback_sync_server" ]
+  }
+
   if (enable_configuration_policy) {
     sources += [
       "driver/sync_policy_handler.cc",
@@ -598,6 +602,40 @@ static_library("sync") {
   configs += [ "//build/config/compiler:wexit_time_destructors" ]
 }
 
+static_library("loopback_sync_server") {
+  sources = [
+    "engine_impl/loopback_server/loopback_connection_manager.cc",
+    "engine_impl/loopback_server/loopback_connection_manager.h",
+    "engine_impl/loopback_server/loopback_server.cc",
+    "engine_impl/loopback_server/loopback_server.h",
+    "engine_impl/loopback_server/loopback_server_entity.cc",
+    "engine_impl/loopback_server/loopback_server_entity.h",
+    "engine_impl/loopback_server/persistent_bookmark_entity.cc",
+    "engine_impl/loopback_server/persistent_bookmark_entity.h",
+    "engine_impl/loopback_server/persistent_permanent_entity.cc",
+    "engine_impl/loopback_server/persistent_permanent_entity.h",
+    "engine_impl/loopback_server/persistent_tombstone_entity.cc",
+    "engine_impl/loopback_server/persistent_tombstone_entity.h",
+    "engine_impl/loopback_server/persistent_unique_client_entity.cc",
+    "engine_impl/loopback_server/persistent_unique_client_entity.h",
+  ]
+
+  configs += [ "//build/config:precompiled_headers" ]
+
+  public_deps = [
+    "//components/sync/protocol",
+  ]
+  deps = [
+    "//base",
+    "//base/third_party/dynamic_annotations",
+    "//components/invalidation/public",
+    "//net",
+    "//url",
+  ]
+
+  configs += [ "//build/config/compiler:wexit_time_destructors" ]
+}
+
 static_library("test_support_sync_core") {
   testonly = true
   sources = [
@@ -944,6 +982,10 @@ source_set("unit_tests") {
     sources -= [ "core/http_bridge_unittest.cc" ]
   }
 
+  if (is_win) {
+    sources += [ "engine_impl/loopback_server/loopback_server_unittest.cc" ]
+  }
+
   if (enable_configuration_policy) {
     sources += [ "driver/sync_policy_handler_unittest.cc" ]
     deps += [