Enable RL sync by default on iOS

ios/chrome/browser/sync/ios_chrome_sync_client.mm

Index: ios/chrome/browser/sync/ios_chrome_sync_client.mm
diff --git a/ios/chrome/browser/sync/ios_chrome_sync_client.mm b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
index ed427b610645eab597492205a563069490c2844e..74ce61a81bae8d99b8b4868afefc9e134626496e 100644
--- a/ios/chrome/browser/sync/ios_chrome_sync_client.mm
+++ b/ios/chrome/browser/sync/ios_chrome_sync_client.mm
@@ -338,10 +338,12 @@ IOSChromeSyncClient::GetSyncBridgeForModelType(syncer::ModelType type) {
           ->GetDeviceInfoSyncBridge()
           ->AsWeakPtr();
     case syncer::READING_LIST: {
-      ReadingListModel* reading_list_model =
-          ReadingListModelFactory::GetForBrowserState(browser_state_);
-      if (reading_list_model)
-        return reading_list_model->GetModelTypeSyncBridge()->AsWeakPtr();
+      if (ReadingListModelFactory::IsReadingListEnabled()) {

[pavely, 2016/11/20 02:31:58]

It is an issue when GetSyncBridgeForModelType returns invalid WeakPtr. During
startup sync engine will post a call to Bridge::OnSyncStarting, passing a
callback. It will block initialization waiting for bridge to call this callback.
If WeakRef is invalid then this call will never happen.

Here is how to observe this:
- Disable reading list through experimental flag
- Start browser, sign in
- Check datatypes' status on about:sync page.

One way to address this is not to register datatype controller in
profile_sync_components_factory_impl.cc based on experiment value.

Another way is to implement custom ModelTypeController for reading list
overriding  DataTypeController::ReadyForStart method to return false if
experiment is disabled:
https://cs.chromium.org/chromium/src/components/sync/driver/data_type_control...

Here is an example of how it is done for autofill walled datatype:
https://cs.chromium.org/chromium/src/components/autofill/core/browser/autofil...

[Olivier, 2016/11/20 10:32:33]

Problem is that ReadingListService is in ios/chrome/browser at the moment, and
the switch is in 	ios/chrome/browser/chrome_switches.h. 
So I cannot easily check the switch, and if I create a custom
DataTypeController, as it will also be in ios/chrome/browser

+sdefresne, our referent components and dependency to see if he has an idea.

+        ReadingListModel* reading_list_model =
+            ReadingListModelFactory::GetForBrowserState(browser_state_);
+        if (reading_list_model)
+          return reading_list_model->GetModelTypeSyncBridge()->AsWeakPtr();
+      }
       return base::WeakPtr<syncer::ModelTypeSyncBridge>();
     }
     default: