A CL that adds a new entry is already up at https://crrev.com/c/1070139. Going forward, we also need to find some way to avoid this happening whenever a new sync model type is added. Maybe we can have a whitelist pattern that covers all (current and future) model types, or if that's not feasible, at least a static_assert.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fd819de0a712442e0679ad982f5e31838ba9b0a9

commit fd819de0a712442e0679ad982f5e31838ba9b0a9
Author: Marc Treib <treib@chromium.org>
Date: Fri May 25 19:39:14 2018

MemoryInfra: Add missing sync model type MOUNTAIN_SHARE

This new model type was introduced in https://crrev.com/c/1068528, which
failed to add the new whitelist entry. Similar to https://crbug.com/818570.

Bug:  846252 
Change-Id: Ie11443d48f62cc034bf177ff0e7d35d06aa6c2c8
Reviewed-on: https://chromium-review.googlesource.com/1070139
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561973}
[modify] https://crrev.com/fd819de0a712442e0679ad982f5e31838ba9b0a9/base/trace_event/memory_infra_background_whitelist.cc

So, we'd like to have something like
static_assert(41 == syncer::MODEL_TYPE_COUNT, "msg");
in memory_infra_background_whitelist.cc, but that won't work: The enum is defined in /components/sync, but the whitelist is in /base. :-/

ssid@, any other ideas?

Maybe adding comment here would work: https://cs.chromium.org/chromium/src/components/browser_sync/profile_sync_service.cc?type=cs&q=syncer::MODEL_TYPE_COUNT&g=0&l=1367

You can add an assert somewhere else (even after the enum definition itself), just mention the file in the assert comment, the point is that something should fail automatically :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5686be59bc4889a0644fe61f913c4fb9fcc8055c

commit 5686be59bc4889a0644fe61f913c4fb9fcc8055c
Author: vitaliii <vitaliii@chromium.org>
Date: Fri Jun 01 08:11:47 2018

[Sync::Cleanup] Add more |static_assert|'s related to new types.

So that it is more difficult to forget to update something when adding a
new sync datatype.

Bug:  846252 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I45a2bb0c39407291229d05c8509127761a993351
Reviewed-on: https://chromium-review.googlesource.com/1078767
Commit-Queue: vitaliii <vitaliii@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563562}
[modify] https://crrev.com/5686be59bc4889a0644fe61f913c4fb9fcc8055c/chrome/browser/sync/profile_sync_service_factory_unittest.cc
[modify] https://crrev.com/5686be59bc4889a0644fe61f913c4fb9fcc8055c/components/browser_sync/profile_sync_service.cc
[modify] https://crrev.com/5686be59bc4889a0644fe61f913c4fb9fcc8055c/components/sync/base/model_type.h
[modify] https://crrev.com/5686be59bc4889a0644fe61f913c4fb9fcc8055c/components/sync/protocol/proto_value_conversions_unittest.cc
[modify] https://crrev.com/5686be59bc4889a0644fe61f913c4fb9fcc8055c/components/sync/syncable/model_type.cc
[modify] https://crrev.com/5686be59bc4889a0644fe61f913c4fb9fcc8055c/ios/chrome/browser/sync/profile_sync_service_factory_unittest.cc

Thanks vitaliii@! I think we can consider this done now.