reland of [sync] Add Sessions integration tests

components/sync/test/fake_server/sessions_hierarchy.h

Index: components/sync/test/fake_server/sessions_hierarchy.h
diff --git a/components/sync/test/fake_server/sessions_hierarchy.h b/components/sync/test/fake_server/sessions_hierarchy.h
index 8d323d499b6fc48f684207e8d83bda2607b6fffe..93199ca5e0c62cdb409d3bffef9ce8549d5bd8b4 100644
--- a/components/sync/test/fake_server/sessions_hierarchy.h
+++ b/components/sync/test/fake_server/sessions_hierarchy.h
@@ -21,6 +21,12 @@ class SessionsHierarchy {
   // Creates an empty (no windows) SessionsHierachy.
   SessionsHierarchy();
 
+  // Copy constructor.
+  SessionsHierarchy(const SessionsHierarchy& other);
+
+  // Creates a SessionsHierarchy with specified set of windows.
+  SessionsHierarchy(std::initializer_list<std::multiset<std::string>> windows);
+
   ~SessionsHierarchy();
 
   // Add a window to the builder with one tab.