toolchain: Update to Rust 1.31.0

We updated the production toolchain from 1.30 to 1.31 in CL:1366446.
This CL does the same upgrade for the local developer toolchain and
Kokoro.

The relevant changes are in rust-toolchain and kokoro/Dockerfile.
The rest are from rustfmt.

TEST=cargo fmt --all -- --check
TEST=as described in kokoro/README.md

Change-Id: I3b4913f3e237baa36c664b4953be360c09efffd4
Reviewed-on: https://chromium-review.googlesource.com/1374376
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: David Tolnay <dtolnay@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/sys_util/src/guest_memory.rs b/sys_util/src/guest_memory.rs
index 3622085..6644ff2 100644
--- a/sys_util/src/guest_memory.rs
+++ b/sys_util/src/guest_memory.rs
@@ -496,7 +496,8 @@ mod tests {
         let gm = GuestMemory::new(&vec![
             (start_region1, size_region1),
             (start_region2, size_region2),
-        ]).unwrap();
+        ])
+        .unwrap();
 
         let mem_size = gm.memory_size();
         assert_eq!(mem_size, size_region1 + size_region2);