[Suggestions] Remove TreeNode.getSuggestionAt() in favor of a visitor.
The method is only used in tests, where we can get better coverage by
using the visitor pattern when verifying section contents (as it allows
e.g. also verifying the specific action of an action item).
This also pushes dependencies on domain-specific code out of the
TreeNode interface.
Also implement stringification for (sub-)trees in terms of a visitor.
BUG=616090
Review-Url: https://codereview.chromium.org/2860463002
Cr-Commit-Position: refs/heads/master@{#474660}
24 files changed
