Replace uses of MUST_USE_RESULT by V8_WARN_UNUSED_RESULT in ElapsedTimer.

src/platform/elapsed-timer.h

 // Copyright 2013 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 69 matching lines @@
     TimeTicks ticks = Now();
     TimeDelta elapsed = ticks - start_ticks_;
     ASSERT(elapsed.InMicroseconds() >= 0);
     start_ticks_ = ticks;
     ASSERT(IsStarted());
     return elapsed;
   }
 
   // Returns the time elapsed since the previous start. This method may only
   // be called on a previously started timer.
-  MUST_USE_RESULT TimeDelta Elapsed() const {
+  TimeDelta Elapsed() const V8_WARN_UNUSED_RESULT {
     ASSERT(IsStarted());
     TimeDelta elapsed = Now() - start_ticks_;
     ASSERT(elapsed.InMicroseconds() >= 0);
     return elapsed;
   }
 
   // Returns |true| if the specified |time_delta| has elapsed since the
   // previous start, or |false| if not. This method may only be called on
   // a previously started timer.
-  MUST_USE_RESULT bool HasExpired(TimeDelta time_delta) const {
+  bool HasExpired(TimeDelta time_delta) const V8_WARN_UNUSED_RESULT {
     ASSERT(IsStarted());
     return Elapsed() >= time_delta;
   }
 
  private:
-  MUST_USE_RESULT V8_INLINE(static TimeTicks Now()) {
+  V8_INLINE(static TimeTicks Now()) V8_WARN_UNUSED_RESULT {
     TimeTicks now = TimeTicks::HighResNow();
     ASSERT(!now.IsNull());
     return now;
   }
 
   TimeTicks start_ticks_;
 #ifdef DEBUG
   bool started_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_PLATFORM_ELAPSED_TIMER_H_