This fixes the computation of the {may_have_interesting_symbols} flag
for the last map computed in {Map::AddMissingTransitions} method. The
last map is allocated ahead of time, but the flag is only correct once
the descriptors are actually installed in the end.

R=bmeurer@chromium.org
TEST=mjsunit/regress/regress-crbug-786020
BUG=chromium:786020

Change-Id: Iff97780609fe596437eb6bea85606a1c3bb2ac4c
Reviewed-on: https://chromium-review.googlesource.com/789839Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49631}