Fix gcc 4.9.2 sign-compare error.

BUG=

Review URL: https://codereview.chromium.org/1393903003

Cr-Commit-Position: refs/heads/master@{#31186}

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);