Refactor OAuthTokenGetterImpl for token-exchange

This is a very simple refactor, to facilitate hooking in a
token-exchange service.

Bug: 954427
Change-Id: If7b7a3528cd38090c64d6ab4e6990793ff5469ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575128
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652450}
2 files changed