[zone] Fix initial position rounding

Fixed: chromium:1279383
Change-Id: I0fb6e15d47698b90c51cc27e18a526ba6d0fc1d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3338656
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78364}

src/zone/zone.cc

@@ -90,9 +90,9 @@ void Zone::Reset() {
   keep->ZapContents();
 
   segment_head_ = keep;
-  position_ = keep->start();
+  position_ = RoundUp(keep->start(), kAlignmentInBytes);
   limit_ = keep->end();
-  DCHECK_EQ(allocation_size(), 0);
+  DCHECK_LT(allocation_size(), kAlignmentInBytes);
   DCHECK_EQ(segment_bytes_allocated_, keep->total_size());
 }