Only walk the heap clearing KeyedStoreICs when the dictionary in question belongs to an object that's used as a prototype.
This is a temporary mitigation until we have a way to clear such ICs without having to walk the heap.

BUG=v8:4335
LOG=y
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1275363002

Cr-Commit-Position: refs/heads/master@{#30076}