These accessors are only called from the Parser, never ParserBase, so
there's no need to expose them in parser-base.h. Instead, access them
through FunctionState::scope().

This also allows removal of the Types::Variable typedefs.

Bug: v8:6460
Change-Id: I01186c53d3cc2a2737f3c07169fdd122dff5b174
Reviewed-on: https://chromium-review.googlesource.com/530034Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45879}