Add the javascript shell WebKit JavaScriptCore as an optional
download in third_party/jsc. This is a CIPD download, and includes
an update.sh script to upload a new version of jsc to CIPD and
update DEPS.
Bug: b/322134579
Change-Id: I94902ccdff3d121e5be51ef6eeab595fc24ad7e3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/348161
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
