Open the LoginDatabase on the DB thread, not the UI thread.

components/password_manager/core/browser/password_store.h

Index: components/password_manager/core/browser/password_store.h
diff --git a/components/password_manager/core/browser/password_store.h b/components/password_manager/core/browser/password_store.h
index b678368cbaa3b742ae3af142ca3a8bdb859b4872..fac936559230cf98dc87ad514ac4778ec7a8b526 100644
--- a/components/password_manager/core/browser/password_store.h
+++ b/components/password_manager/core/browser/password_store.h
@@ -51,6 +51,11 @@ class PasswordSyncableService;
 // Interface for storing form passwords in a platform-specific secure way.
 // The login request/manipulation API is not threadsafe and must be used
 // from the UI thread.
+// Implementations, however, should carry out most tasks asynchronously on a
+// background thread: the base class provides functionality to facilitate this.
+// I/O heavy initialization should also be performed asynchronously in this
+// manner. If this deferred initialization fails, all subsequent method calls
+// should fail without side effects, return no data, and send no notifications.
 // PasswordStoreSync is a hidden base class because only PasswordSyncableService
 // needs to access these methods.
 class PasswordStore : protected PasswordStoreSync,