We would return the wrong chunk for the first element past the chunk
boundary, e.g. if the first chunk was size=8, then Find(8) would
return an address in the first block rather than the second one.

Bug: v8:8077
Change-Id: I90281f853dd7ca68dc065ed773d0ae9787f00988
Reviewed-on: https://chromium-review.googlesource.com/1183483
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55294}