[stubs] Properly handle read-only properties in StoreGlobalViaContextStub.

[stubs] Properly handle read-only properties in StoreGlobalViaContextStub.

We don't need the hole check and slow runtime mode for read-only
properties this way.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/1255133002

Cr-Commit-Position: refs/heads/master@{#29881}