Add SyncScheduler for scheduling CryptAuth enrollments and syncing devices.

components/proximity_auth/cryptauth/BUILD.gn

Index: components/proximity_auth/cryptauth/BUILD.gn
diff --git a/components/proximity_auth/cryptauth/BUILD.gn b/components/proximity_auth/cryptauth/BUILD.gn
index 92f35843a0c8127534a2b49fc3a90f4f3c319ff4..43c56435efe88569acd0f2658293b8ab18e5aaff 100644
--- a/components/proximity_auth/cryptauth/BUILD.gn
+++ b/components/proximity_auth/cryptauth/BUILD.gn
@@ -21,6 +21,10 @@ source_set("cryptauth") {
     "cryptauth_enrollment_utils.h",
     "secure_message_delegate.cc",
     "secure_message_delegate.h",
+    "sync_scheduler.cc",
+    "sync_scheduler.h",
+    "sync_scheduler_impl.cc",
+    "sync_scheduler_impl.h",
   ]
 
   deps = [
@@ -65,6 +69,7 @@ source_set("unit_tests") {
     "cryptauth_client_impl_unittest.cc",
     "cryptauth_enroller_impl_unittest.cc",
     "fake_secure_message_delegate_unittest.cc",
+    "sync_scheduler_impl_unittest.cc",
   ]
 
   deps = [