[sync] Add test coverage for session tabs without header

This scenario verifies that, if a user ends up with corrupt data
consisting of tabs without their corresponding header entity,
nevertheless those tabs will eventually be subject to garbage
collection.

Change-Id: I50cec71a46e02731580b2c1ddb2a448533a0757c
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431024
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Victor Vianna <victorvianna@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810563}

chrome/browser/sync/test/integration/single_client_sessions_sync_test.cc

@@ -710,6 +710,54 @@ IN_PROC_BROWSER_TEST_F(SingleClientSessionsSyncTest,
                                          /*LOCAL_DELETION=*/0));
 }
 
+IN_PROC_BROWSER_TEST_F(SingleClientSessionsSyncTest,
+                       GarbageCollectionOfForeignOrphanTabWithoutHeader) {
+  const std::string kForeignSessionTag = "ForeignSessionTag";
+  const SessionID kWindowId = SessionID::FromSerializedValue(5);
+  const SessionID kTabId1 = SessionID::FromSerializedValue(1);
+  const SessionID kTabId2 = SessionID::FromSerializedValue(2);
+  const base::Time kLastModifiedTime =
+      base::Time::Now() - base::TimeDelta::FromDays(100);
+
+  SessionSyncTestHelper helper;
+
+  // There are two orphan tab entities without a header entity.
+
+  sync_pb::EntitySpecifics tab1;
+  *tab1.mutable_session() =
+      helper.BuildTabSpecifics(kForeignSessionTag, kWindowId, kTabId1);
+
+  sync_pb::EntitySpecifics tab2;
+  *tab2.mutable_session() =
+      helper.BuildTabSpecifics(kForeignSessionTag, kWindowId, kTabId2);
+
+  for (const sync_pb::EntitySpecifics& specifics : {tab1, tab2}) {
+    GetFakeServer()->InjectEntity(
+        syncer::PersistentUniqueClientEntity::CreateFromSpecificsForTesting(
+            /*non_unique_name=*/"",
+            sync_sessions::SessionStore::GetClientTag(specifics.session()),
+            specifics,
+            /*creation_time=*/syncer::TimeToProtoTime(kLastModifiedTime),
+            /*last_modified_time=*/syncer::TimeToProtoTime(kLastModifiedTime)));
+  }
+
+  base::HistogramTester histogram_tester;
+  ASSERT_TRUE(SetupSync()) << "SetupSync() failed.";
+
+  // Verify that all entities have been deleted.
+  WaitForHierarchyOnServer(SessionsHierarchy());
+
+  std::vector<sync_pb::SyncEntity> entities =
+      fake_server_->GetSyncEntitiesByModelType(syncer::SESSIONS);
+  for (const sync_pb::SyncEntity& entity : entities) {
+    EXPECT_NE(kForeignSessionTag, entity.specifics().session().session_tag());
+  }
+
+  EXPECT_EQ(
+      2, histogram_tester.GetBucketCount("Sync.ModelTypeEntityChange3.SESSION",
+                                         /*LOCAL_DELETION=*/0));
+}
+
 // Regression test for crbug.com/915133 that verifies the browser doesn't crash
 // if the server sends corrupt data during initial merge.
 IN_PROC_BROWSER_TEST_F(SingleClientSessionsSyncTest, CorruptInitialForeignTab) {