Snapshots don't yet work in 64-bit win/cross builds, so disable them
until they do.

No behavior change in builds that aren't 64-bit win/cross builds.

Bug: chromium:803591
Change-Id: I7a04c7e01a58a95a2bfb78c7d2593c7c5c5041cf
Reviewed-on: https://chromium-review.googlesource.com/936668Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51554}