reland of [sync] Add Sessions integration tests

components/sync/test/fake_server/sessions_hierarchy.cc

Index: components/sync/test/fake_server/sessions_hierarchy.cc
diff --git a/components/sync/test/fake_server/sessions_hierarchy.cc b/components/sync/test/fake_server/sessions_hierarchy.cc
index 8cdde963ba802551540c0f4bc7de620695675e1e..204c8cd694cb640a8be403d4318b6e81a2435525 100644
--- a/components/sync/test/fake_server/sessions_hierarchy.cc
+++ b/components/sync/test/fake_server/sessions_hierarchy.cc
@@ -10,12 +10,16 @@ namespace fake_server {
 
 SessionsHierarchy::SessionsHierarchy() {}
 
+SessionsHierarchy::SessionsHierarchy(const SessionsHierarchy& other) = default;
+
+SessionsHierarchy::SessionsHierarchy(
+    std::initializer_list<std::multiset<std::string>> windows)
+    : windows_(windows) {}
+
 SessionsHierarchy::~SessionsHierarchy() {}
 
 void SessionsHierarchy::AddWindow(const std::string& tab) {
-  SessionsHierarchy::Window window;
-  window.insert(tab);
-  windows_.insert(window);
+  windows_.insert({tab});
 }
 
 void SessionsHierarchy::AddWindow(const std::multiset<std::string>& tabs) {