At least under some circumstances (GCC 6 with x64 Linux for me),
the code in `serializer-for-background-compilation.cc` guarded by
`ENABLE_SLOW_DCHECKS` attempts to use `std::iterator_traits` on the
`FunctionalList`, which previously failed compilation because
the standard iterator member types were unavailable.
This adds these members.

Refs: https://chromium-review.googlesource.com/c/v8/v8/+/1800578

Change-Id: Ifece423fce31d98777c6a65ef442623c321ecba3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1921800Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65014}