Added pretty printers for some Ink Drop enums.

ui/views/animation/ink_drop_state.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/animation/ink_drop_state.h"
 
 #include <string>
 
 #include "base/logging.h"
 
 namespace views {
 
-std::string ToString(InkDropState state) {
+std::string ToString(const InkDropState& state) {
   switch (state) {
     case InkDropState::HIDDEN:
       return std::string("HIDDEN");
     case InkDropState::ACTION_PENDING:
       return std::string("ACTION_PENDING");
     case InkDropState::ACTION_TRIGGERED:
       return std::string("ACTION_TRIGGERED");
     case InkDropState::ALTERNATE_ACTION_PENDING:
       return std::string("ALTERNATE_ACTION_PENDING");
     case InkDropState::ALTERNATE_ACTION_TRIGGERED:
       return std::string("ALTERNATE_ACTION_TRIGGERED");
     case InkDropState::ACTIVATED:
       return std::string("ACTIVATED");
     case InkDropState::DEACTIVATED:
       return std::string("DEACTIVATED");
   }
   NOTREACHED()
       << "Should never be reached but is necessary for some compilers.";
   return std::string("UNKNOWN");
 }
 
 }  // namespace views