With this CL SloppyArguments immediately go to dictionary elements on
deletion, keeping the arguments backing store packed.

Bug: v8:6251
Change-Id: I90d1972179447bf6810e7fe2b8e0bc8703b38d9d
Reviewed-on: https://chromium-review.googlesource.com/486921Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45286}