Rename DatabaseTaskSynchronizer to TaskSynchronizer and move it to platform/

Source/modules/webdatabase/DatabaseContext.cpp

Index: Source/modules/webdatabase/DatabaseContext.cpp
diff --git a/Source/modules/webdatabase/DatabaseContext.cpp b/Source/modules/webdatabase/DatabaseContext.cpp
index 60359af8e68d51408463bc5ea964f22392ef92dc..c953463715226db3f55bb45275262b0e32174f4f 100644
--- a/Source/modules/webdatabase/DatabaseContext.cpp
+++ b/Source/modules/webdatabase/DatabaseContext.cpp
@@ -215,7 +215,7 @@ void DatabaseContext::stopDatabases()
     // DatabaseThread.
 
     if (m_databaseThread && !m_hasRequestedTermination) {
-        DatabaseTaskSynchronizer sync;
+        TaskSynchronizer sync;
         m_databaseThread->requestTermination(&sync);
         m_hasRequestedTermination = true;
         sync.waitForTaskCompletion();