[mojo] Update message_quota_checker.h doc in prep for base::PostTask migration

R=rockot@google.com

Bug: 1026641
Change-Id: Id9e75f1959687ff9cae1fe92e2923f7f38eff8ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219152
Commit-Queue: Gabriel Charette <gab@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#772453}

mojo/public/cpp/bindings/lib/message_quota_checker.h

@@ -31,10 +31,10 @@ namespace internal {
 // outgoing queue. Additionally, |BeforeWrite()| should be called immediately
 // before writing each message to the corresponding message pipe.
 //
-// Also note that messages posted to a different sequence with
-// |base::PostTask()| and the like, need to be treated as locally queued. Task
-// queues can grow arbitrarily long, and it's ideal to perform unread quota
-// checks before posting.
+// Also note that messages posted to a different sequence with base::ThreadPool
+// and the like, need to be treated as locally queued. Task queues can grow
+// arbitrarily long, and it's ideal to perform unread quota checks before
+// posting.
 //
 // Either |BeforeMessagesEnqueued()| or |BeforeWrite()| may cause the quota
 // to be exceeded, thus invoking the |maybe_crash_function| set in this