sync: Add --enable-sync-sessions-v2 and necessary plumbing to use SessionsSyncManager.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index cbf190faacae28253aafbb2f10119bc1725a7e1f..1790b229877c5e38d6cf77e6b5004b3bbdfeb253 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -766,6 +766,9 @@ const char kEnableSuggestionsTabPage[]      = "enable-suggestions-ntp";
 const char kEnableSyncSyncedNotifications[] =
     "enable-sync-synced-notifications";
 
+// Enables sync/API based session sync implementation (in favor of legacy).
+const char kEnableSyncSessionsV2[] = "enable-sync-sessions-v2";
+
 // Enables synced articles.
 const char kEnableSyncArticles[] = "enable-sync-articles";