Fix gcc 4.9.2 sign-compare error.

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index ff6f3d2b41be6b8f1a326bf82e73963c8357cee9..69a8d89fccba5c5aa06f91d2a1995870b70ef033 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -264,7 +264,7 @@ class Bitmap {
   // Clears all bits starting from {cell_base_index} up to and excluding
   // {index}. Note that {cell_base_index} is required to be cell aligned.
   void ClearRange(uint32_t cell_base_index, uint32_t index) {
-    DCHECK_EQ(IndexInCell(cell_base_index), 0);
+    DCHECK_EQ(IndexInCell(cell_base_index), 0u);
     DCHECK_GE(index, cell_base_index);
     uint32_t start_cell_index = IndexToCell(cell_base_index);
     uint32_t end_cell_index = IndexToCell(index);