Rename DatabaseTaskSynchronizer to TaskSynchronizer and move it to platform/

Source/modules/webdatabase/Database.cpp

Index: Source/modules/webdatabase/Database.cpp
diff --git a/Source/modules/webdatabase/Database.cpp b/Source/modules/webdatabase/Database.cpp
index 4f7e39fd325755878e68d0315f844722498248ae..5880d3f816ccaffefa9c8d4a2504e413a5344d17 100644
--- a/Source/modules/webdatabase/Database.cpp
+++ b/Source/modules/webdatabase/Database.cpp
@@ -219,7 +219,7 @@ Vector<String> Database::tableNames()
     // FIXME: Not using isolatedCopy on these strings looks ok since threads take strict turns
     // in dealing with them. However, if the code changes, this may not be true anymore.
     Vector<String> result;
-    DatabaseTaskSynchronizer synchronizer;
+    TaskSynchronizer synchronizer;
     if (!databaseContext()->databaseThread() || databaseContext()->databaseThread()->terminationRequested(&synchronizer))
         return result;