Add chromecast/internal to the visibility list of libxml.

Bug: internal b/144557463
Bug: 1101110
Test: Build
Change-Id: I39549934fb7234b58c664337acd05232d60d0875
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265434
Commit-Queue: Junbo Ke <juke@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#784224}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0a502516f4814e5c41eb66b2bc3dc157a1ff5035