Add CompoundSyncable class for synchronizing containers of Syncables.

blimp/helium/BUILD.gn

Index: blimp/helium/BUILD.gn
diff --git a/blimp/helium/BUILD.gn b/blimp/helium/BUILD.gn
index 528af28aeaaf186586d1625cdfc272f7f5efe5b3..0f1951ee97591144978430f06c1f54fe249038a3 100644
--- a/blimp/helium/BUILD.gn
+++ b/blimp/helium/BUILD.gn
@@ -7,6 +7,8 @@ component("helium") {
   sources = [
     "coded_value_serializer.cc",
     "coded_value_serializer.h",
+    "compound_syncable.cc",
+    "compound_syncable.h",
     "errors.h",
     "lww_register.h",
     "result.cc",
@@ -48,11 +50,11 @@ source_set("unit_tests") {
 
   sources = [
     "coded_value_serializer_unittest.cc",
+    "compound_syncable_unittest.cc",
     "helium_test.h",
     "lww_register_unittest.cc",
     "result_unittest.cc",
     "revision_generator_unittest.cc",
-    "syncable_unittest.cc",
     "version_vector_unittest.cc",
   ]