Walk the VariableMap instead of the ast.

Change-Id: I03ee9145230bcbfe04c5e31dc8d8b3a98a00a4be
Reviewed-on: https://chromium-review.googlesource.com/c/1424865
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58968}