Sync cleanup: Remove DataTypeController::REGISTRATION_IGNORED

Contrary to the code comment, it was only used for the PROXY_TABS data
type. This is now handled explicitly instead.

Bug: 647505
Change-Id: I5fd3fdb4d93230acac32966c1ee54334983dc6b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462093
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816148}

components/sync/driver/data_type_controller.h

@@ -43,10 +43,8 @@ class DataTypeController : public base::SupportsWeakPtr<DataTypeController> {
 
   // Returned from RegisterWithBackend.
   enum RegisterWithBackendResult {
-    // Used when RegisterWithBackend is called on an already-registered type.
-    // TODO(crbug.com/647505): Get rid of this entry entirely if possible.
-    REGISTRATION_IGNORED,
-    // Indicates that the initial download for this type is already complete.
+    // Indicates that the initial download for this type is already complete, or
+    // wasn't needed in the first place (e.g. for proxy types).
     TYPE_ALREADY_DOWNLOADED,
     // Indicates that the initial download for this type still needs to be done.
     TYPE_NOT_YET_DOWNLOADED,

components/sync/driver/data_type_manager_impl.cc

@@ -206,10 +206,11 @@ void DataTypeManagerImpl::RegisterTypesWithBackend() {
       // time.
       DCHECK(!data_type_status_table_.GetFailedTypes().Has(dtc->type()));
       switch (dtc->RegisterWithBackend(configurer_)) {
-        case DataTypeController::REGISTRATION_IGNORED:
-          break;
         case DataTypeController::TYPE_ALREADY_DOWNLOADED:
-          downloaded_types_.Put(type);
+          // Proxy types (as opposed to protocol types) don't actually have any
+          // data, so keep proxy types out of |downloaded_types_|.
+          if (!IsProxyType(type))
+            downloaded_types_.Put(type);
           break;
         case DataTypeController::TYPE_NOT_YET_DOWNLOADED:
           downloaded_types_.Remove(type);

components/sync_sessions/proxy_tabs_data_type_controller.cc

@@ -45,7 +45,8 @@ ProxyTabsDataTypeController::RegisterWithBackend(
   state_ = RUNNING;
   state_changed_cb_.Run(state_);
 
-  return REGISTRATION_IGNORED;
+  // Proxy types don't have any data to download.
+  return TYPE_ALREADY_DOWNLOADED;
 }
 
 void ProxyTabsDataTypeController::Stop(syncer::ShutdownReason shutdown_reason,