This switches the current win32 bots to win32 under the hood in MB. We'll
remove them and replace them with win64 bots in a follow up on the infra
side.

This also infers the clang option from asan, because on windows we need
to set clang explicitly.

TBR=sergiyb@chromium.org

Bug: chromium:786303
Change-Id: I9dddd5050a21a364c302a761ff15ddd21e97c7dc
Reviewed-on: https://chromium-review.googlesource.com/883103Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50828}