See https://google.github.io/styleguide/cppguide.html#Enumerator_Names

Also rename "FunctionBody" to "FunctionBodyType" and move it inside
Parser, which is the only place it's referenced.

R=caitp@igalia.com, littledan@chromium.org

Review-Url: https://codereview.chromium.org/2245133003
Cr-Commit-Position: refs/heads/master@{#38671}