[heap] Fix -Wsign-compare warnings

test/unittests/heap/slot-set-unittest.cc

 // Copyright 2016 the V8 project 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 <limits>
 #include <map>
 
 #include "src/globals.h"
 #include "src/heap/slot-set.h"
 #include "src/heap/spaces.h"
@@ skipping 109 matching lines @@
   }
 }
 
 TEST(SlotSet, RemoveRange) {
   CheckRemoveRangeOn(0, Page::kPageSize);
   CheckRemoveRangeOn(1 * kPointerSize, 1023 * kPointerSize);
   for (uint32_t start = 0; start <= 32; start++) {
     CheckRemoveRangeOn(start * kPointerSize, (start + 1) * kPointerSize);
     CheckRemoveRangeOn(start * kPointerSize, (start + 2) * kPointerSize);
     const uint32_t kEnds[] = {32, 64, 100, 128, 1024, 1500, 2048};
-    for (int i = 0; i < sizeof(kEnds) / sizeof(uint32_t); i++) {
+    for (size_t i = 0; i < sizeof(kEnds) / sizeof(uint32_t); i++) {
       for (int k = -3; k <= 3; k++) {
         uint32_t end = (kEnds[i] + k);
         if (start < end) {
           CheckRemoveRangeOn(start * kPointerSize, end * kPointerSize);
         }
       }
     }
   }
   SlotSet set;
   set.SetPageStart(0);
@@ skipping 71 matching lines @@
           CHECK(reinterpret_cast<uintptr_t>(host_addr) < start ||
                 reinterpret_cast<uintptr_t>(host_addr) >= end);
           return KEEP_SLOT;
         },
         TypedSlotSet::KEEP_EMPTY_CHUNKS);
   }
 }
 
 }  // namespace internal
 }  // namespace v8