This CL adds a SetProperty method to the KeyedStoreGenericGenerator
that mirrors what "KeyedStoreGeneric" does (used for
KeyedStoreIC_MegaMorphic). This new SetProperty method is then used
in the SetProperty stub.

Change-Id: I72a684238ef6c3b8c4db8ba957d5b79238f7e495
Reviewed-on: https://chromium-review.googlesource.com/1164945
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55005}