[heap] Fix -Wsign-compare warnings

test/cctest/heap/test-spaces.cc

Index: test/cctest/heap/test-spaces.cc
diff --git a/test/cctest/heap/test-spaces.cc b/test/cctest/heap/test-spaces.cc
index 262d0c5d5810f1bac5cf963de92f8cde603e71c1..b354df2e3e8e53bf1167ff07bb74d7ffcb539420 100644
--- a/test/cctest/heap/test-spaces.cc
+++ b/test/cctest/heap/test-spaces.cc
@@ -530,7 +530,8 @@ TEST(SizeOfInitialHeap) {
   }
 
   // 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) {