Refactor cast_mojo gn args so that the cast renderer can be enabled with ENABLE_CAST_RENDERER

Bug: b/126906251
Test: Verified that mojo_media args did not change.
Change-Id: I167fde3c1cea718c87d03d3dc2c3b61964e77988
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512677
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Alex Leung <alexleung@google.com>
Cr-Commit-Position: refs/heads/master@{#641363}
4 files changed