[ic] Use validity cells to protect keyed element stores against object's prototype chain modifications.

... instead of clearing of all the KeyedStoreICs which didn't always work.

BUG=chromium:662907, v8:5561
TBR=verwaest@chromium.org, bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2534613002
Cr-Commit-Position: refs/heads/master@{#41332}