- Avoid invoking Trace() for in-construction objects as the method may
  access uninitialized fields, e.g., fields that have bogus state with
  zeroed memory like std::list.
- Conservatively scan in-construction objects for pointers.
- Verify that stack scan indeed finds all in-construction objects that
  are present on the heap and vice versa.

Bug: chromium:1056170
Change-Id: I2c68da2b8072f715b5a0dcdb1202d5f874c6c6e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2388106Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69670}