[turbofan] Make context specialization into a reducer.

src/compiler/js-context-specialization.h

Index: src/compiler/js-context-specialization.h
diff --git a/src/compiler/js-context-specialization.h b/src/compiler/js-context-specialization.h
index b8b50ed6c36a1dbbf0f2a575892e8328c4500f36..1c0b54ffb0e5cb5acbcaf838a5a1abaa3db8cfe5 100644
--- a/src/compiler/js-context-specialization.h
+++ b/src/compiler/js-context-specialization.h
@@ -16,12 +16,14 @@ namespace compiler {
 
 // Specializes a given JSGraph to a given context, potentially constant folding
 // some {LoadContext} nodes or strength reducing some {StoreContext} nodes.
-class JSContextSpecializer {
+class JSContextSpecializer : public Reducer {
  public:
   JSContextSpecializer(CompilationInfo* info, JSGraph* jsgraph, Node* context)
       : info_(info), jsgraph_(jsgraph), context_(context) {}
 
-  void SpecializeToContext();
+  virtual Reduction Reduce(Node* node) OVERRIDE;
+
+  // Visible for unit testing.
   Reduction ReduceJSLoadContext(Node* node);
   Reduction ReduceJSStoreContext(Node* node);