Migrate Mojo tests from legacy Callback to OnceCallback

This CL flips `use_once_callback` flag on the Mojo code generator for
Mojo tests, and fixes all compile errors after that.
After this CL, Mojo interfaces there service starts using
base::OnceCallback instead of base::Callback on its return value
handling.

Bug: 714018
Change-Id: Iaa9566292500cefd92ecedd137ed4ba4bfec502e
Reviewed-on: https://chromium-review.googlesource.com/1156190
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579717}
25 files changed