Convert PlatformTestHelper::Factory to RepeatingCallback with Miscellaneous Cleanup

BUG=714018,940130
TBR=sky@chromium.org

Change-Id: I6fe5a4d82d299bc0676a802bc42c28086dc796bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526742
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641405}
2 files changed