Make copy_cdb_to_output depend on vs_toolchain.py

Ideally, copy_cdb_to_output should depend on the files it's copying. But
since those live outside Chromium that's not really practical. Depending
on vs_toolchain.py is probably good enough; the important part is this
will force copy_cdb_to_output to run whenever the VS version changes.

Bug: 846313
Change-Id: Ie0596fb820ba71c40b65325d663f95c2bffe30a4
Reviewed-on: https://chromium-review.googlesource.com/1073451
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561893}

build/win/BUILD.gn

@@ -21,6 +21,7 @@ if (is_win) {
     script = "//build/win/copy_cdb_to_output.py"
     inputs = [
       script,
+      "//build/vs_toolchain.py",
     ]
     outputs = [
       "$root_out_dir/cdb/cdb.exe",