Init ModelTypeController's delegate to nullptr

The initial value of ModelTypeController::delegate_ is never read
because the delegate is only used once the controller starts running.
However, let's make sure it's initially null to detect CHECK failures if
there is a violation of the API.

Bug: 885019
Change-Id: I4a5623986b2bbd6c96a02f218438367d8e78a7fc
Reviewed-on: https://chromium-review.googlesource.com/1238729
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593529}

components/sync/driver/model_type_controller.cc

@@ -50,7 +50,7 @@ SyncStopMetadataFate TakeStrictestMetadataFate(SyncStopMetadataFate fate1,
 ModelTypeController::ModelTypeController(
     ModelType type,
     std::unique_ptr<ModelTypeControllerDelegate> delegate_on_disk)
-    : DataTypeController(type), state_(NOT_RUNNING) {
+    : DataTypeController(type) {
   delegate_map_.emplace(ConfigureContext::STORAGE_ON_DISK,
                         std::move(delegate_on_disk));
 }

components/sync/driver/model_type_controller.h

@@ -76,10 +76,10 @@ class ModelTypeController : public DataTypeController {
       delegate_map_;
 
   // State of this datatype controller.
-  State state_;
+  State state_ = NOT_RUNNING;
 
   // Owned by |delegate_map_|. Null while NOT_RUNNING.
-  ModelTypeControllerDelegate* delegate_;
+  ModelTypeControllerDelegate* delegate_ = nullptr;
 
   // Callback for use when starting the datatype (usually MODEL_STARTING, but
   // STOPPING if abort requested while starting).