devtools: download shell binaries and set origin when run w/ --mojo-version.

Before, a user of devtools that wanted to use prebuilt mojo at particular
version, had to:

 - set up DEPS hooks and use GN build rules that together downloaded and copied
   the shell and the network service at the correct version to the output
   directory
 - set the --origin to point to deployed mojo services at particular version

This patch adds a --mojo-version flag that does all of the above, downloading
binaries as needed and setting the origin accordingly.

Once consumers switch over to --mojo-version, we can retire the GN rules for
downloading prebuilt shells.

Fixes https://github.com/domokit/devtools/issues/62.
Fixes https://github.com/domokit/devtools/issues/63.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1844943004 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: da012592f169e8c531e4a92cdf00d3c39413dd34