Fix lint error in 3.1 branch.

src/hydrogen-instructions.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 773 matching lines @@
   virtual Representation RequiredInputRepresentation(int index) const {
     return Representation::None();
   }
 
   DECLARE_CONCRETE_INSTRUCTION(BlockEntry, "block_entry")
 };
 
 
 class HDeoptimize: public HControlInstruction {
  public:
-  HDeoptimize(int environment_length)
+  explicit HDeoptimize(int environment_length)
       : HControlInstruction(NULL, NULL),
         values_(environment_length) { }
 
   virtual Representation RequiredInputRepresentation(int index) const {
     return Representation::None();
   }
 
   virtual int OperandCount() { return values_.length(); }
   virtual HValue* OperandAt(int index) { return values_[index]; }
 
@@ skipping 2702 matching lines @@
   HValue* object() { return left(); }
   HValue* key() { return right(); }
 };
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_