Disable snapshots in 64-bit win/cross builds for now.

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}

gni/v8.gni

@@ -33,7 +33,9 @@ declare_args() {
 
   # Enable the snapshot feature, for fast context creation.
   # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
-  v8_use_snapshot = true
+  # TODO(thakis): Make snapshots work in 64-bit win/cross builds,
+  # https://803591
+  v8_use_snapshot = !(is_win && host_os != "win" && target_cpu == "x64")
 
   # Use external files for startup data blobs:
   # the JS builtins sources and the start snapshot.