Add PostTaskAndReplyWithResult to migration mappings

TBR=robliao@chromium.org

Bug: Documentation update
Change-Id: Ib2c5b090b8c8187e6ca8cffb20f2efcde17bbf53
Reviewed-on: https://chromium-review.googlesource.com/581635Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488752}

docs/task_scheduler_migration.md

@@ -104,6 +104,8 @@ assigned files by:
 * base::ThreadChecker -> base::SequenceChecker
   * ThreadChecker::CalledOnValidThread() -> DCHECK_CALLED_ON_VALID_SEQUENCE(...)
 * base::ThreadLocalStorage::Slot -> base::SequenceLocalStorageSlot
+* BrowserThread::PostTaskAndReplyWithResult() -> base::PostTaskAndReplyWithResult()
+  (from post_task.h or from task_runner_util.h (if you need to feed a TaskRunner))
 * BrowserThread::DeleteOnThread -> base::DeleteOnTaskRunner / base::RefCountedDeleteOnSequence
 * CreateSingleThreadTaskRunnerWithTraits() -> CreateSequencedTaskRunnerWithTraits()
    * Every CreateSingleThreadTaskRunnerWithTraits() usage should be accompanied