Fix presubmit error with explicit constructor in test.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/462813003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23078 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/cctest/compiler/test-simplified-lowering.cc

@@ -614,7 +614,7 @@ class TestingGraph : public HandleAndZoneScope, public GraphAndBuilders {
   Node* end;
   Node* ret;
 
-  TestingGraph(Type* p0_type, Type* p1_type = Type::None())
+  explicit TestingGraph(Type* p0_type, Type* p1_type = Type::None())
       : GraphAndBuilders(main_zone()),
         typer(main_zone()),
         jsgraph(graph(), common(), &typer) {