GitLab

- Move global win-only flags from //build/config/compiler to
  //build/config/win:compiler which the former already references if is_win

- Move /FC from the toolchain to //build/config/win:compiler too: The
  toolchain isn't where random compiler flags should be added.

- This allows moving msvc_use_absolute_paths from a gni file to a gn file,
  so do that as well.

No intended behavior change.

Bug: none
Change-Id: I1b35a4242649105e005b94df911849ebf44d7c5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541636
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644799}