Fixes an invalid cast of an Operator1<StorePropertyParameters> object
to Operator1<LanguageMode> by reading the language mode from
StorePropertyParameters. This code happened to work before because
StorePropertyParameters's first field has type LanguageMode.

Cleanup for cfi_vptr=1; see https://www.chromium.org/developers/testing/control-flow-integrity

BUG=chromium:457523
R=bmeurer@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1311393008

Cr-Commit-Position: refs/heads/master@{#30493}