Properly print the new context types in HeapObjectShortPrint() so we see
what kind of context it is, and the number of context slots.

Bug: v8:7570, v8:7588
Change-Id: I43414996fd953f77e140e8750b2dc4923c9bce95
Reviewed-on: https://chromium-review.googlesource.com/986135Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52297}