DOM Storage: Use exclusive locking when migrating data from SQLite.

Bug: 1000959
Change-Id: Ia0bed864089e14ad413a409fb646706b17cc96fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359232
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#798724}

components/services/storage/dom_storage/legacy_dom_storage_database.cc

@@ -155,9 +155,13 @@ bool LegacyDomStorageDatabase::LazyOpen(bool create_if_needed) {
     return false;
   }
 
-  db_.reset(new sql::Database());
+  db_ = std::make_unique<sql::Database>();
   db_->set_histogram_tag("DOMStorageDatabase");
 
+  // This database should only be accessed from the process hosting the storage
+  // service, so exclusive locking is appropriate.
+  db_->set_exclusive_locking();
+
   // This db does not use [meta] table, store mmap status data elsewhere.
   db_->set_mmap_alt_status();