Tidy up ThreadState::removeInterruptor() assert.

Source/platform/heap/ThreadState.cpp

 /*
  * Copyright (C) 2013 Google Inc. 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
@@ skipping 1088 matching lines @@
     }
 }
 
 void ThreadState::removeInterruptor(Interruptor* interruptor)
 {
     checkThread();
     SafePointScope scope(HeapPointersOnStack, SafePointScope::AllowNesting);
     {
         MutexLocker locker(threadAttachMutex());
         size_t index = m_interruptors.find(interruptor);
-        RELEASE_ASSERT(index >= 0);
+        RELEASE_ASSERT(index != kNotFound);
         m_interruptors.remove(index);
     }
 }
 
 void ThreadState::Interruptor::onInterrupted()
 {
     ThreadState* state = ThreadState::current();
     ASSERT(state);
     ASSERT(!state->isAtSafePoint());
     state->safePoint(HeapPointersOnStack);
@@ skipping 41 matching lines @@
             return gcInfo;
         }
     }
     if (needLockForIteration)
         threadAttachMutex().unlock();
     return nullptr;
 }
 #endif
 
 } // namespace blink