Remaining changes to build all code in src.git with -Wimplicit-fallthrough.

Fixes a missing break that was accidentally deleted in
https://codereview.chromium.org/2701003002/diff/140001/components/sync/syncable/model_type.cc
which caused each SUPERVISED_USER_SHARED_SETTINGS to also receive an
ARTICLES default value.

This CL was uploaded by git cl split.

R=zea@chromium.org

Bug: 177475,806921
Change-Id: Iaa5b42a8d2ac5787b787b6163adb2f3f7407e6c8
Reviewed-on: https://chromium-review.googlesource.com/891746
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nicolas Zea <zea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533017}

components/sync/syncable/model_type.cc

@@ -236,6 +236,7 @@ void AddDefaultFieldValue(ModelType type, sync_pb::EntitySpecifics* specifics) {
       break;
     case SUPERVISED_USER_SHARED_SETTINGS:
       specifics->mutable_managed_user_shared_setting();
+      break;
     case ARTICLES:
       specifics->mutable_article();
       break;