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
[email protected],[email protected],[email protected]

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

Cr-Commit-Position: refs/heads/master@{#327755}
diff --git a/base/single_thread_task_runner.h b/base/single_thread_task_runner.h
index b5311db..6e93193 100644
--- a/base/single_thread_task_runner.h
+++ b/base/single_thread_task_runner.h
@@ -16,7 +16,8 @@
 // there is a specific need to run tasks on only a single thread.
 //
 // SingleThreadTaskRunner implementations might:
-//   - Post tasks to an existing thread's MessageLoop (see MessageLoopProxy).
+//   - Post tasks to an existing thread's MessageLoop (see
+//     MessageLoop::task_runner()).
 //   - Create their own worker thread and MessageLoop to post tasks to.
 //   - Add tasks to a FIFO and signal to a non-MessageLoop thread for them to
 //     be processed. This allows TaskRunner-oriented code run on threads