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Fix post-commit comments from gab@ in crrev.com/c/1261816:
- s/task_environment_.GetMainThreadTaskRunner/ThreadTaskRunnerHandle::Get/.
- std::move for pointers.
- C++11-style init for ScopedTaskEnviroment for UI and IO.

R=gab@chromium.org,jochen@chromium.org

BUG=892139,891670

Change-Id: I8403c53b34a5cfc2ef358eee982de88b60ec74cc
Reviewed-on: https://chromium-review.googlesource.com/c/1286144Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600834}