Win asan needs separate "clang" config as clang is not the default on windows.

TBR=sergiyb@chromium.org
NOTRY=true

Bug: chromium:726584
Change-Id: Ic49eedf9a229eb3f81774c8d03c9d19ba1211c3d
Reviewed-on: https://chromium-review.googlesource.com/720801Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48576}