[Extensions] More bindings improvements

Explicitly freeze the schema in chrome, pass safe arguments to GetAvailability,
and broaden test access checks.

BUG=604901
BUG=603725
BUG=591164

Review URL: https://codereview.chromium.org/1899973002

Cr-Commit-Position: refs/heads/master@{#388353}
(cherry picked from commit 585b125ef7168c104631e23ee5cad0108c838f52)

Review URL: https://codereview.chromium.org/1930163004 .

Cr-Commit-Position: refs/branch-heads/2704@{#314}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
4 files changed