win/mac/ios: Don't copy asan runtime around in non-asan sanitizer builds.

From what I can tell, it's not intentional that we keyed this off
using_sanitizer instead of is_asan.

As far as I know, the only sanitizer we currently use on mac and ios is
asan, so this should be a no-op there.

On Windows, we also have experimental CFI bots, and this might help
the 32-bit CFI bots that now no longer build the asan runtime.
(But presumably CFI needs some other runtime that we also no longer
build -- but it'll hopefully move the error a bit further back,
and copying the asan runtimes is needless work.)

TBR=pcc

Bug: 968434
Change-Id: I56528f2e82852d8c3145d657db77833321a6602c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637032
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664745}