Add support for unary plus in hydrogen compiler.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index 235b669933294a6d890ba0d099d2f77d41d11e2f..2c79dc29fff0bbd9e1c508b9778fdca6a231d6f1 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -222,6 +222,7 @@ class HGraph: public ZoneObject {
     undefined_constant_.set(constant);
   }
   HConstant* GetConstantUndefined() const { return undefined_constant_.get(); }
+  HConstant* GetConstant0();
   HConstant* GetConstant1();
   HConstant* GetConstantMinus1();
   HConstant* GetConstantTrue();
@@ -283,6 +284,7 @@ class HGraph: public ZoneObject {
   ZoneList<HValue*> values_;
   ZoneList<HPhi*>* phi_list_;
   SetOncePointer<HConstant> undefined_constant_;
+  SetOncePointer<HConstant> constant_0_;
   SetOncePointer<HConstant> constant_1_;
   SetOncePointer<HConstant> constant_minus1_;
   SetOncePointer<HConstant> constant_true_;