Store device salt callback instead of resource context in WebRTC Audio Private API

      The resource context pointer was used only to get access to the device
ID salt callback. However, the resource context pointer can become dangling and cause
use-after-free issues.

BUG=572224

Review URL: https://codereview.chromium.org/1692913003

Cr-Commit-Position: refs/heads/master@{#375259}