ENVIRONMENT and STATS
For syncing clients: iOS 11, iPhone X
From chrome: Chromium dev
From about:sync

REPRO STEPS
log in with an account with data saved, wait ~30 seconfs

ACTUAL RESULTS
Hitting  `DCHECK(AreValidSpecifics(specifics));` 
log:
```
[0808/154009.493588:FATAL:session_store.cc(388)] Check failed: AreValidSpecifics(specifics). 
[0808/154009.493932:FATAL:session_store.cc(39)] Check failed: tab_node_id > TabNodePool::kInvalidTabNodeID (-1 vs. -1)
[0808/154009.494056:FATAL:client_tag_based_model_type_processor.cc(923)] Check failed: data.client_tag_hash == GetHashForTag(bridge_->GetClientTag(data)) ({0} vs. {1})
[0808/154009.494160:FATAL:client_tag_based_model_type_processor.cc(907)] Check failed: !bridge_->SupportsGetStorageKey() || !storage_key.empty(). 
[0808/154009.501754:FATAL:session_store.cc(388)] Check failed: AreValidSpecifics(specifics). 
[0808/154009.501853:FATAL:session_store.cc(39)] Check failed: tab_node_id > TabNodePool::kInvalidTabNodeID (-1 vs. -1)
[0808/154009.501925:FATAL:client_tag_based_model_type_processor.cc(923)] Check failed: data.client_tag_hash == GetHashForTag(bridge_->GetClientTag(data)) ({0} vs. {1})
[0808/154009.501989:FATAL:client_tag_based_model_type_processor.cc(907)] Check failed: !bridge_->SupportsGetStorageKey() || !storage_key.empty(). 
[0808/154009.526226:FATAL:session_store.cc(421)] Check failed: success. 
[0808/154009.539688:FATAL:session_store.cc(421)] Check failed: success. 
[0808/154009.561874:FATAL:client_tag_based_model_type_processor.cc(819)] Check failed: AllStorageKeysPopulated(). 
2018-08-08 15:40:09.654427+0200 Chromium[19624:2791927] stat on /var/mobile/Containers/Data/Application/A2FF2744-8487-493D-A785-AB17579E317F/Library/Application Support/Chromium/Default/Bookmarks: No such file or directory
[0808/154010.072009:FATAL:client_tag_based_model_type_processor.cc(604)] Check failed: AllStorageKeysPopulated(). 
```

EXPECTED RESULTS
No DCHECK should be hit

ADDITIONAL INFO
Anything else which may help us debug the issue; screenshots of error
messages or states are always helpful.