Implement OAuth token-exchange via gRPC.

This CL makes a gRPC call to the UpdateRobotToken service to get a
new OAuth code, and exchanges it for an access token.

Bug: 954427
Change-Id: I95754ca5ed056edee75ee5fce1368e6fbfa01a40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621355
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661597}
4 files changed