[turbofan]: Small visualizer cleanup and fix for string handling

src/compiler/graph-visualizer.h

Index: src/compiler/graph-visualizer.h
diff --git a/src/compiler/graph-visualizer.h b/src/compiler/graph-visualizer.h
index 611351244beaa5925e0a46a9d7e093e0fe79395e..17094c23c57c37a93433565bbfba91aa193f1c97 100644
--- a/src/compiler/graph-visualizer.h
+++ b/src/compiler/graph-visualizer.h
@@ -20,6 +20,8 @@ class RegisterAllocator;
 class Schedule;
 class SourcePositionTable;
 
+FILE* OpenVisualizerLogFile(CompilationInfo* info, const char* phase,
+                            const char* suffix, const char* mode);
 
 struct AsDOT {
   explicit AsDOT(const Graph& g) : graph(g) {}