This removes a stability check from CanInlineArrayResizeOperation. This
is safe, since all prototype maps are either stable or in dictionary
mode, where the latter is ruled out separately.

Bug: v8:7127, v8:7205
Change-Id: I3a07805cb89f41adf031c6c2a6dd5b2defcb87a8
Reviewed-on: https://chromium-review.googlesource.com/848778
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50353}