Revert "[offthread] Serialize new-space objects into old space"

This reverts commit 8c8e6b41.

Reason for revert: Makes Mac64 debug quite flaky: https://ci.chromium.org/p/v8/builders/ci/V8%20Mac64%20-%20debug/28766

Original change's description:
> [offthread] Serialize new-space objects into old space
> 
> If an object made it into serialization, it probably deserves to be in
> old space when it is deserialized. This will also make off-thread
> deserialization simpler, as there is no off-thread new space.
> 
> Bug: chromium:1075999
> Change-Id: Icabf2f0ae0a3e0205a1094dd0ffe675e69bd1d8e
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184291
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#67606}

TBR=ulan@chromium.org,jgruber@chromium.org,leszeks@chromium.org

Change-Id: I0baca3c3b7985fcda6b9055f93111ec99c207d29
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1075999
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2183925Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67615}

src/snapshot/serializer.cc

@@ -569,16 +569,11 @@ SnapshotSpace GetSnapshotSpace(HeapObject object) {
     // Large code objects are not supported and cannot be expressed by
     // SnapshotSpace.
     DCHECK_NE(heap_space, CODE_LO_SPACE);
-    switch (heap_space) {
-      // Young generation objects are tenured, as objects that have survived
-      // until snapshot building probably deserve to be considered 'old'.
-      case NEW_SPACE:
-        return SnapshotSpace::kOld;
-      case NEW_LO_SPACE:
-        return SnapshotSpace::kLargeObject;
-
-      default:
-        return static_cast<SnapshotSpace>(heap_space);
+    // Young generation large objects are tenured.
+    if (heap_space == NEW_LO_SPACE) {
+      return SnapshotSpace::kLargeObject;
+    } else {
+      return static_cast<SnapshotSpace>(heap_space);
     }
   }
 }