This CL changes "MessageWriter" type to std::function instead of a
plain function pointer. This allows capturing lambdas, which in turn
are used to make unittests more robust.

R=sigurds@chromium.org

Bug: v8:8880
Change-Id: I9d71ddcac173af36e5b62852f2a9ec6dcfac9f78
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1640201
Commit-Queue: Simon Zünd <szuend@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61958}