This cleans up a few things:
 * refactors a few remaining missed EPs
 * removes unnecessary includes
 * rename one Context entry point
 * moves some ES2 EP validation to validationES2

The last item makes for a significant diff, but this is a refactor
change only, with no functionality change.

BUG=angleproject:747

Change-Id: I7860cc4b6260b6c22faa5f2885297333c0cdb4ed
Reviewed-on: https://chromium-review.googlesource.com/483426Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>