Revert "Fix sync Nigori cache GUID left empty upon mismatch"

This reverts commit cd858876.

Reason for revert: a more aggressive logic can be implemented and
is safer against users that are already in a broken state (empty
cache GUID).

Original change's description:
> Fix sync Nigori cache GUID left empty upon mismatch
> 
> In the error-handling codepath where a mismatch is found on the cache
> GUID, when sync is starting for Nigori, the new cache GUID should be
> stored in the ModelTypeState. Otherwise, it would remain empty, which
> means the next sync start the mismatch detection doesn't take place.
> 
> This can explain cache-GUID mismatches being ignored, and bad
> consequences such as violating the sync protocol by sending a non-empty
> progress marker despite the sync birthday being empty.
> 
> Change-Id: I48d4b35a61dd20f56c416779bee686e1d582606d
> Bug: 1063021
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111090
> Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
> Auto-Submit: Mikel Astiz <mastiz@chromium.org>
> Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
> Cr-Commit-Position: refs/heads/master@{#751966}

TBR=mastiz@chromium.org,mmoskvitin@google.com,rushans@google.com

Change-Id: I2de46c2cd71e30f99b3a953c815de8f78d3a568c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1063021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2112631Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752028}

components/sync/nigori/nigori_model_type_processor.cc

@@ -390,11 +390,6 @@ bool NigoriModelTypeProcessor::IsConnectedForTest() const {
   return IsConnected();
 }
 
-const sync_pb::ModelTypeState&
-NigoriModelTypeProcessor::GetModelTypeStateForTest() {
-  return model_type_state_;
-}
-
 bool NigoriModelTypeProcessor::IsTrackingMetadata() {
   return model_type_state_.initial_sync_done();
 }
@@ -417,14 +412,13 @@ void NigoriModelTypeProcessor::ConnectIfReady() {
     return;
   }
 
-  if (model_type_state_.has_cache_guid() &&
-      model_type_state_.cache_guid() != activation_request_.cache_guid) {
+  if (!model_type_state_.has_cache_guid()) {
+    model_type_state_.set_cache_guid(activation_request_.cache_guid);
+  } else if (model_type_state_.cache_guid() != activation_request_.cache_guid) {
     ClearMetadataAndReset();
     DCHECK(model_ready_to_sync_);
   }
 
-  model_type_state_.set_cache_guid(activation_request_.cache_guid);
-
   // Cache GUID verification earlier above guarantees the user is the same.
   model_type_state_.set_authenticated_account_id(
       activation_request_.authenticated_account_id.ToString());

components/sync/nigori/nigori_model_type_processor.h

@@ -59,7 +59,6 @@ class NigoriModelTypeProcessor : public ModelTypeProcessor,
   bool IsTrackingMetadata() override;
 
   bool IsConnectedForTest() const;
-  const sync_pb::ModelTypeState& GetModelTypeStateForTest();
 
  private:
   // Returns true if the handshake with sync thread is complete.

components/sync/nigori/nigori_model_type_processor_unittest.cc

@@ -527,8 +527,6 @@ TEST_F(NigoriModelTypeProcessorTest, ShouldResetDataOnCacheGuidMismatch) {
   processor()->OnSyncStarting(request, base::DoNothing());
 
   EXPECT_FALSE(processor()->IsTrackingMetadata());
-  EXPECT_EQ(processor()->GetModelTypeStateForTest().cache_guid(),
-            kOtherCacheGuid);
 
   EXPECT_FALSE(ProcessorHasEntity());