[signin] Fix crash in AccountReconcilor::Lock::~Lock()

The lock can now outlive the AccountReconcilor.

Bug: 923094
Change-Id: I92443549d50354eb1418fe8495dddea5bf310d5d
Reviewed-on: https://chromium-review.googlesource.com/c/1425499Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625150}

components/signin/core/browser/account_reconcilor.cc

@@ -158,14 +158,15 @@ std::string PickFirstGaiaAccount(
 }  // namespace
 
 AccountReconcilor::Lock::Lock(AccountReconcilor* reconcilor)
-    : reconcilor_(reconcilor) {
+    : reconcilor_(reconcilor->weak_factory_.GetWeakPtr()) {
   DCHECK(reconcilor_);
   reconcilor_->IncrementLockCount();
 }
 
 AccountReconcilor::Lock::~Lock() {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
-  reconcilor_->DecrementLockCount();
+  if (reconcilor_)
+    reconcilor_->DecrementLockCount();
 }
 
 AccountReconcilor::ScopedSyncedDataDeletion::ScopedSyncedDataDeletion(

components/signin/core/browser/account_reconcilor.h

@@ -53,7 +53,7 @@ class AccountReconcilor : public KeyedService,
     ~Lock();
 
    private:
-    AccountReconcilor* reconcilor_;
+    base::WeakPtr<AccountReconcilor> reconcilor_;
     THREAD_CHECKER(thread_checker_);
     DISALLOW_COPY_AND_ASSIGN(Lock);
   };

components/signin/core/browser/account_reconcilor_unittest.cc

@@ -2782,3 +2782,10 @@ TEST_F(AccountReconcilorTest, ScopedSyncedDataDeletionDestructionOrder) {
   DeleteReconcilor();
   // data_deletion is destroyed after the reconcilor, this should not crash.
 }
+
+TEST_F(AccountReconcilorTest, LockDestructionOrder) {
+  AccountReconcilor* reconcilor = GetMockReconcilor();
+  AccountReconcilor::Lock lock(reconcilor);
+  DeleteReconcilor();
+  // |lock| is destroyed after the reconcilor, this should not crash.
+}