Transient network conditions can cause the CIPD client download to fail,
so add a retry to the download.

As part of adding the retry, also extracted the download code to its own
function to ensure the WebClient is disposed promptly.

Bug: 1145741
Change-Id: I66bb5cde7ec4f637dd1161252c2b096af6abb8a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2519730Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>