Contain static_cast in RealtimeAudioDestinationHandler to Context()

We have an architectural problem that needs to be resolved to avoid
static_casts in WebAudio.  As a temporary measure, contain usage to a
single method to avoid noise during code reviews.

Change-Id: I0c63bd80ffc6eabd02bbce3df5db6c38cd633861
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5365945


Reviewed-by: Sina Firoozabadi <sinafirooz@chromium.org>
Commit-Queue: Michael Wilson <mjwilson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1272986}