SyncableServiceBasedModelTypeController: Handle null SyncableService

It turns out that in some cases, the SyncableService passed into
SyncableServiceBasedModelTypeController may be null - in particular,
this may happen for HistoryDeleteDirectivesModelTypeController if the
HistoryService is null (which can happen e.g. if its database
initialization failed).
This CL fixes the issue by handling null within
SyncableServiceBasedModelTypeController's ControllerDelegate.

(cherry picked from commit e145a12)

Bug: 1394815
Change-Id: I959ed706c3feb12b318b5861866a945b392195eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4075804
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1078663}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4083728
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Srinivas Sista <srinivassista@chromium.org>
Owners-Override: Srinivas Sista <srinivassista@chromium.org>
Cr-Commit-Position: refs/branch-heads/5359@{#1104}
Cr-Branched-From: 27d3765-refs/heads/main@{#1058933}

components/sync/driver/syncable_service_based_model_type_controller.cc

@@ -26,7 +26,8 @@ class ControllerDelegate : public ModelTypeControllerDelegate {
       : type_(type), dump_stack_(dump_stack) {
     DCHECK(store_factory);
 
-    // The |syncable_service| can be null in tests.
+    // The |syncable_service| can be null in some cases (e.g. when the
+    // underlying service failed to initialize), and in tests.
     if (syncable_service) {
       bridge_ = std::make_unique<SyncableServiceBasedBridge>(
           type_, std::move(store_factory),
@@ -43,24 +44,43 @@ class ControllerDelegate : public ModelTypeControllerDelegate {
 
   void OnSyncStarting(const DataTypeActivationRequest& request,
                       StartCallback callback) override {
+    if (!bridge_) {
+      // TODO(crbug.com/1394815): Consider running `callback` here to avoid
+      // blocking the Sync machinery.
+      return;
+    }
     GetBridgeDelegate()->OnSyncStarting(request, std::move(callback));
   }
 
   void OnSyncStopping(SyncStopMetadataFate metadata_fate) override {
+    if (!bridge_) {
+      return;
+    }
     GetBridgeDelegate()->OnSyncStopping(metadata_fate);
   }
 
   void GetAllNodesForDebugging(AllNodesCallback callback) override {
+    if (!bridge_) {
+      // TODO(crbug.com/1394815): Consider running `callback` here.
+      return;
+    }
     GetBridgeDelegate()->GetAllNodesForDebugging(std::move(callback));
   }
 
   void GetTypeEntitiesCountForDebugging(
       base::OnceCallback<void(const TypeEntitiesCount&)> callback)
       const override {
+    if (!bridge_) {
+      // TODO(crbug.com/1394815): Consider running `callback` here.
+      return;
+    }
     GetBridgeDelegate()->GetTypeEntitiesCountForDebugging(std::move(callback));
   }
 
   void RecordMemoryUsageAndCountsHistograms() override {
+    if (!bridge_) {
+      return;
+    }
     GetBridgeDelegate()->RecordMemoryUsageAndCountsHistograms();
   }