[Cast MRP] Eliminated media_source_helper.{cc,h}

Change-Id: If36c8e11912078057c8143d2c8ed8ecbc86c9f0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606969
Commit-Queue: John Williams <jrw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662902}
38 files changed