[node] remove everything before updating v8.

This ensures that we have a clean slate when updating to newer V8.

TBR=machenbach@chromium.org

Bug: v8:8584
Change-Id: I37f6e4a42738e5e9ea5bfdca5465d6a3e984fe65
Reviewed-on: https://chromium-review.googlesource.com/c/1378169Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58242}

tools/node/update_node.py

@@ -91,6 +91,7 @@ def UpdateTarget(repository, options, files_to_keep):
     git_args.append(["add"] + files_to_keep)            # add and commit
     git_args.append(["commit", "-m", "keep files"])     # files we want to keep
 
+  git_args.append(["clean", "-fxd"])                    # nuke everything else
   git_args.append(["remote", "add", "source", source])  # point to source repo
   git_args.append(["fetch", "source", "HEAD"])          # sync to current branch
   git_args.append(["checkout", "-f", "FETCH_HEAD"])     # switch to that branch