Fix keyed access of primitive objects in the runtime.

Fix keyed access of primitive objects in the runtime.
For now it uses a pretty slow path for accessing strings by wrapping it into a new temporary wrapper.

BUG=v8:4042, v8:3088
LOG=y

Review URL: https://codereview.chromium.org/1221303019

Cr-Commit-Position: refs/heads/master@{#29576}