The DCHECK related to a time when dictionary mode prototypes were the payload
of complex data driven handlers. Now the additional data is used to hold
entirely different kinds of objects. The DCHECK made no sense anymore. Cleaning
up the names makes this clearer.

Bug: chromium:986187
Change-Id: I7173d7d2824396c04c01acb4ceb74693ee9ce6b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1724215
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62993}