Encouraging ThreadTaskRunnerHandle usage in net/dns module

MessageLoopProxy is deprecated.
This basically does a search and replace of:

MessageLoopProxy::current() -> ThreadTaskRunnerHandle::Get().

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045

Review URL: https://codereview.chromium.org/1137433003

Cr-Commit-Position: refs/heads/master@{#331916}
3 files changed