[Sync] Put session tracker in charge of maintaining local state.

components/sync_sessions/synced_session_tracker.cc

Index: components/sync_sessions/synced_session_tracker.cc
diff --git a/components/sync_sessions/synced_session_tracker.cc b/components/sync_sessions/synced_session_tracker.cc
index a74b877c085c11e890af5c51567516596c8d01be..52385d70fafa5bf4cbbd906c6ce699223a97f91b 100644
--- a/components/sync_sessions/synced_session_tracker.cc
+++ b/components/sync_sessions/synced_session_tracker.cc
@@ -339,6 +339,48 @@ sessions::SessionTab* SyncedSessionTracker::GetTabImpl(
   return tab_ptr;
 }
 
+bool SyncedSessionTracker::ReassociateTab(const std::string& session_tag,
+                                          SessionID::id_type old_tab_id,
+                                          SessionID::id_type new_tab_id) {
+  DCHECK_NE(TabNodePool::kInvalidTabID, old_tab_id);
+  DCHECK_NE(TabNodePool::kInvalidTabID, new_tab_id);
+
+  sessions::SessionTab* tab_ptr = nullptr;
+  auto old_tab_iter = synced_tab_map_[session_tag].find(old_tab_id);
+  if (old_tab_iter == synced_tab_map_[session_tag].end())
+    return false;
+
+  // It's possible a placeholder is already in place for the new tab. If that's
+  // the case, reuse the placeholder SessionTab. Otherwise, reuse the old
+  // SessionTab.
+  auto new_tab_iter = synced_tab_map_[session_tag].find(new_tab_id);
+  if (new_tab_iter != synced_tab_map_[session_tag].end()) {
+    tab_ptr = new_tab_iter->second;
+  } else {
+    tab_ptr = old_tab_iter->second;
+  }
+
+  // If the old tab is unmapped, update the tab id under which it is indexed.
+  auto unmapped_tabs_iter = unmapped_tabs_[session_tag].find(old_tab_id);
+  if (unmapped_tabs_iter != unmapped_tabs_[session_tag].end()) {
+    std::unique_ptr<sessions::SessionTab> tab =
+        std::move(unmapped_tabs_iter->second);
+    unmapped_tabs_[session_tag].erase(unmapped_tabs_iter);
+    unmapped_tabs_[session_tag][new_tab_id] = std::move(tab);
+  }
+
+  // Update the tab id.
+  DVLOG(1) << "Remapped tab " << old_tab_id << " to tab " << new_tab_id;
+  tab_ptr->tab_id.set_id(new_tab_id);
+
+  // Remove the tab from the synced tab map under the old id.
+  synced_tab_map_[session_tag].erase(old_tab_iter);
+
+  // Add the tab back into the tab map with the new id.
+  synced_tab_map_[session_tag][new_tab_id] = tab_ptr;
+  return true;
+}
+
 void SyncedSessionTracker::Clear() {
   // Cleanup unmapped tabs and windows.
   unmapped_windows_.clear();