This makes sure the inline allocation of generator objects only shrinks
initial maps when slack tracking is actually in progress. Shrinking all
unused properties unconditionally is bogus because instances using them
might have become unreachable and collected by the GC.

R=mvstanton@chromium.org
TEST=mjsunit/regress/regress-crbug-741078
BUG=chromium:741078

Change-Id: Iaf2f08a4fa82c820a945bf012d24c760a6b4f514
Reviewed-on: https://chromium-review.googlesource.com/567982Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46585}