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

Bug: v8:6251
Change-Id: I2afa4fb5f0af9942eee0a1606942f5f289539330
Reviewed-on: https://chromium-review.googlesource.com/480379
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44857}