Cleans up the internalization. Also, clean up no-longer-used ast
symbols, iterator and hasInstance, which were left behind after other
refactors. Having an enum here should keep this clean in the future.

Change-Id: Id526784b0361c7a2242b21ecf2af72b0403c6ad8
Reviewed-on: https://chromium-review.googlesource.com/440204Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43069}