Oilpan: add missing leftmost trace()s for GC mixins.

Source/web/DatabaseClientImpl.cpp

Index: Source/web/DatabaseClientImpl.cpp
diff --git a/Source/web/DatabaseClientImpl.cpp b/Source/web/DatabaseClientImpl.cpp
index 254735290ce94f12b1e6a8fc2e422837fe046a64..adc26896da0aab8009831f4246ca734e3d47a5b8 100644
--- a/Source/web/DatabaseClientImpl.cpp
+++ b/Source/web/DatabaseClientImpl.cpp
@@ -47,6 +47,11 @@ DatabaseClientImpl::~DatabaseClientImpl()
 {
 }
 
+void DatabaseClientImpl::trace(Visitor* visitor)
+{
+    DatabaseClient::trace(visitor);
+}
+
 bool DatabaseClientImpl::allowDatabase(ExecutionContext* executionContext, const String& name, const String& displayName, unsigned long estimatedSize)
 {
     ASSERT(executionContext->isContextThread());