remoting: Migrate callers of message_loop_proxy() to task_runner()

Migrate callers of {MessageLoop,Thread}::message_loop_proxy() to
{MessageLoop,Thread}::task_runner(). Since the types at the call sites
have already been updated, this is just a bulk rename.

BUG=465354

Review URL: https://codereview.chromium.org/1181213004

Cr-Commit-Position: refs/heads/master@{#334212}
28 files changed