AI: Use SecureContext for languageModel API

This change updates languageModel API to use `SecureContext`.
Missed in https://crrev.com/c/6020818 due to rename.

Bug: 372311600
Change-Id: If4ea0c117da38e17b2b33df9b0020bf8218b41f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6027212
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1384649}

Author: ayuishii