[Sync] Refactor sync manager into interface.

sync/sync.gyp

Index: sync/sync.gyp
diff --git a/sync/sync.gyp b/sync/sync.gyp
index b1554f556c5001d31e0d4fc4e8cbe090ebb3f9c0..f4286fb1712abf145ae61360b740ec62fc041698 100644
--- a/sync/sync.gyp
+++ b/sync/sync.gyp
@@ -55,6 +55,8 @@
         'internal_api/public/util/immutable.h',
         'internal_api/public/util/syncer_error.cc',
         'internal_api/public/util/syncer_error.h',
+        'internal_api/public/util/sync_string_conversions.cc',
+        'internal_api/public/util/sync_string_conversions.h',
         'internal_api/public/util/report_unrecoverable_error_function.h',
         'internal_api/public/util/unrecoverable_error_handler.h',
         'internal_api/public/util/unrecoverable_error_info.h',
@@ -302,6 +304,8 @@
         'internal_api/public/read_node.h',
         'internal_api/public/read_transaction.h',
         'internal_api/public/sync_manager.h',
+        'internal_api/public/sync_manager.cc',
+        'internal_api/public/sync_manager_factory.h',
         'internal_api/public/user_share.h',
         'internal_api/public/write_node.h',
         'internal_api/public/write_transaction.h',
@@ -323,7 +327,9 @@
         'internal_api/syncapi_internal.h',
         'internal_api/syncapi_server_connection_manager.cc',
         'internal_api/syncapi_server_connection_manager.h',
-        'internal_api/sync_manager.cc',
+        'internal_api/sync_manager_factory.cc',
+        'internal_api/sync_manager_impl.cc',
+        'internal_api/sync_manager_impl.h',
         'internal_api/user_share.cc',
         'internal_api/write_node.cc',
         'internal_api/write_transaction.cc',