[test] Correctly restore allocator on SequentialUnmapperTest teardown.

The SequentialUnmapperTest replaces the process-global PageAllocator
with a wrapper which tracks allocations.  The suite was deleting the
tracking allocator without first restoring the original PageAllocator,
causing any subsequent tests which tried to use it to use-after-free.

Bug: chromium:934932
Change-Id: I0f69b6a07542a3f381724afdbfb2e9b67a9f39de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2010113
Commit-Queue: Wez <wez@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65872}

test/unittests/heap/unmapper-unittest.cc

@@ -242,6 +242,10 @@ class SequentialUnmapperTest : public TestWithIsolate {
     TestWithIsolate::TearDownTestCase();
     i::FLAG_concurrent_sweeping = old_flag_;
     CHECK(tracking_page_allocator_->IsEmpty());
+
+    // Restore the original v8::PageAllocator and delete the tracking one.
+    CHECK_EQ(tracking_page_allocator_,
+             SetPlatformPageAllocatorForTesting(old_page_allocator_));
     delete tracking_page_allocator_;
     tracking_page_allocator_ = nullptr;
   }