Drive-by-cleanup:
- Add InternalElementsAccessor to expose protected instance methods
  to ElementsAccessor subclasses.
- Make some more ElementsAccessor methods protected that take the
  raw entry as parameter.

Bug: chromium:798644
Change-Id: Iffd00f1953461e8dd22c123e62298410fb6e049c
Reviewed-on: https://chromium-review.googlesource.com/856816
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50480}