[ptr-compr] Fix assert in LayoutDescriptor which failed with 31-bit Smis.

Bug: v8:7703, chromium:876696
Change-Id: Ida3243414215b2ef75a9875ca31cf5a68274f7e0
Reviewed-on: https://chromium-review.googlesource.com/1185186Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55383}

src/layout-descriptor.cc

@@ -243,7 +243,7 @@ LayoutDescriptor* LayoutDescriptor::Trim(Heap* heap, Map* map,
       CalculateCapacity(map, descriptors, num_descriptors);
   // It must not become fast-mode descriptor here, because otherwise it has to
   // be fast pointer layout descriptor already but it's is slow mode now.
-  DCHECK_LT(kSmiValueSize, layout_descriptor_length);
+  DCHECK_LT(kBitsInSmiLayout, layout_descriptor_length);
 
   // Trim, clean and reinitialize this slow-mode layout descriptor.
   int new_backing_store_length =