cros_setup_toolchains: Update list of post cross packages.

Add rust and cargo to list of post cross packages.
This lets them being pulled as part of binary package updates.

BUG=chromium:820330
TEST=cros_setup_toolchains --show-packages host-post-cross
     show rust and cargo.

Change-Id: I8f42ac45650bac2256626e649c70eefffdcc3b27
Reviewed-on: https://chromium-review.googlesource.com/1011655
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index f74a3a3..73ee292 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -67,6 +67,8 @@
 # the cross-compilers to be installed first (because they need them to actually
 # build), so we have to delay their installation.
 HOST_POST_CROSS_PACKAGES = (
+    'dev-lang/rust',
+    'dev-util/cargo',
     'virtual/target-sdk-post-cross',
 )