Only the error cases of overwriting readonly properties need the
language_mode to decide whether to throw or be silent. Reading it
from the feedback vector's metadata (just like the C++ code in
ic.cc does) removes the need to duplicate each stub for each
language_mode ("StoreIC" + "StoreICStrict" etc.).

Change-Id: Ic0c67f9d40ca36c65e41b4f162b2ab70d155e549
Reviewed-on: https://chromium-review.googlesource.com/647373Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47836}