Enable tab sync on all platforms by default.

chrome/browser/sync/profile_sync_components_factory_impl.cc

Index: chrome/browser/sync/profile_sync_components_factory_impl.cc
diff --git a/chrome/browser/sync/profile_sync_components_factory_impl.cc b/chrome/browser/sync/profile_sync_components_factory_impl.cc
index c5ef8d3acc38b0108fed9601bd00174e96855279..9dd3fc902d1f49730e04e3f6c040edac7764e82c 100644
--- a/chrome/browser/sync/profile_sync_components_factory_impl.cc
+++ b/chrome/browser/sync/profile_sync_components_factory_impl.cc
@@ -154,10 +154,10 @@ void ProfileSyncComponentsFactoryImpl::RegisterDataTypes(
         new SearchEngineDataTypeController(this, profile_, pss));
   }
 
-  // Session sync is disabled by default, but has some logic to automatically
-  // enable.  Register only if explicitly enabled and not explicitly disabled.
-  if (command_line_->HasSwitch(switches::kEnableSyncTabs) &&
-      !command_line_->HasSwitch(switches::kDisableSyncTabs)) {
+  // Session sync is enabled by default.  Register unless explicitly disabled.
+  // TODO(dubroy): Once this change is permanent, clean up the logic for
+  // explicitly enabling tab sync (http://crbug.com/118570).
+  if (!command_line_->HasSwitch(switches::kDisableSyncTabs)) {
     pss->RegisterDataTypeController(
         new SessionDataTypeController(this, profile_, pss));
   }