[heap] Fix -Wsign-compare warnings

test/cctest/heap/test-spaces.cc

 // 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 512 matching lines @@
   // Executing the empty script gets by with the same number of pages, i.e.,
   // requires no extra space.
   CompileRun("/*empty*/");
   for (int i = FIRST_PAGED_SPACE; i <= LAST_PAGED_SPACE; i++) {
     // Debug code can be very large, so skip CODE_SPACE if we are generating it.
     if (i == CODE_SPACE && i::FLAG_debug_code) continue;
     CHECK_EQ(page_count[i], isolate->heap()->paged_space(i)->CountTotalPages());
   }
 
   // No large objects required to perform the above steps.
-  CHECK_EQ(initial_lo_space, isolate->heap()->lo_space()->Size());
+  CHECK_EQ(initial_lo_space,
+           static_cast<size_t>(isolate->heap()->lo_space()->Size()));
 }
 
 static HeapObject* AllocateUnaligned(NewSpace* space, int size) {
   AllocationResult allocation = space->AllocateRawUnaligned(size);
   CHECK(!allocation.IsRetry());
   HeapObject* filler = NULL;
   CHECK(allocation.To(&filler));
   space->heap()->CreateFillerObjectAt(filler->address(), size,
                                       ClearRecordedSlots::kNo);
   return filler;
@@ skipping 247 matching lines @@
   HeapObject* filler =
       HeapObject::FromAddress(array->address() + array->Size());
   CHECK_EQ(filler->map(), CcTest::heap()->two_pointer_filler_map());
 
   const size_t shrinked = page->ShrinkToHighWaterMark();
   CHECK_EQ(0, shrinked);
 }
 
 }  // namespace internal
 }  // namespace v8