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

This is a reland of 8c8e6b41

Timeouts seem unrelated.

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

Bug: chromium:1075999
Change-Id: Iac4e4eebeca1c343250269cdaad17e23645e9e2b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184970Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67620}

src/snapshot/serializer.cc

@@ -569,11 +569,16 @@ SnapshotSpace GetSnapshotSpace(HeapObject object) {
     // Large code objects are not supported and cannot be expressed by
     // SnapshotSpace.
     DCHECK_NE(heap_space, CODE_LO_SPACE);
-    // Young generation large objects are tenured.
-    if (heap_space == NEW_LO_SPACE) {
-      return SnapshotSpace::kLargeObject;
-    } else {
-      return static_cast<SnapshotSpace>(heap_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);
     }
   }
 }