For BuiltinContinuationFrames we stored the frame-height as a plain
value, which did not cause problems because height (in bytes) always
looks like a Smi to the GC. This CL introduces proper Smi encoding.

Bug: v8:7584
Change-Id: I5e3c41c2e576926bbb2bad7dbe0f277cc017b495
Reviewed-on: https://chromium-review.googlesource.com/1000782Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52476}