base: Remove most uses of MessageLoopProxy

Replace most usage of MessageLoopProxy under base/ with SingleThreadTaskRunner
and ThreadTaskRunnerHandle (excluding the implementation of MessageLoopProxy
itself which will removed later).

This patch was mostly autogenerated with
https://codereview.chromium.org/1010073002.

Depends on https://codereview.chromium.org/1086733002/.

BUG=465354
TBR=nkostylev@chromium.org,pkasting@chromium.org,pauljensen@chromium.org

Review URL: https://codereview.chromium.org/1100773004

Cr-Commit-Position: refs/heads/master@{#327755}
65 files changed