ART: Print C1vis header only if visualizer enabled

Setting kStringFilter currently suppresses graph dumps of non-matching
methods but their headers are still printed. This fixes the issue.

Change-Id: Ib33fb20fcca2bf409534a824e7f76f1feb85724d
diff --git a/compiler/optimizing/graph_visualizer.cc b/compiler/optimizing/graph_visualizer.cc
index 49c0d38..4c28378 100644
--- a/compiler/optimizing/graph_visualizer.cc
+++ b/compiler/optimizing/graph_visualizer.cc
@@ -337,13 +337,11 @@
 
 HGraphVisualizer::HGraphVisualizer(std::ostream* output,
                                    HGraph* graph,
-                                   const CodeGenerator& codegen,
-                                   const char* method_name)
-  : output_(output), graph_(graph), codegen_(codegen) {
-  if (output == nullptr) {
-    return;
-  }
+                                   const CodeGenerator& codegen)
+  : output_(output), graph_(graph), codegen_(codegen) {}
 
+void HGraphVisualizer::PrintHeader(const char* method_name) const {
+  DCHECK(output_ != nullptr);
   HGraphVisualizerPrinter printer(graph_, *output_, "", true, codegen_);
   printer.StartTag("compilation");
   printer.PrintProperty("name", method_name);