[base] Make PostTask destination explicit in /components/webdata

Task APIs v3 will all have an explicit destination. In the past
"default traits" meant ThreadPool. These CLs are thus a no-op
except for the few callers that clearly wanted to post a QuitClosure()
on the current thread (posting a QuitClosure() to a parallel task will
merely bounce it back to the current thread anyways). Those few callers
were migrated to ThreadTaskRunnerHandle::Get().

These changes are thus required to phase out task APIs v2 (post_task.h)
Design doc: https://docs.google.com/document/d/1tssusPykvx3g0gvbvU4HxGyn3MjJlIylnsH13-Tv6s4/edit

This specific change is a subset of
https://chromium-review.googlesource.com/c/chromium/src/+/2216733

This CL was uploaded by git cl split.

R=pkasting@chromium.org

Bug: 1026641
Change-Id: Id38d941acd0e77333cdcef9c34a3eb509650755c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215917
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772279}

components/webdata/common/web_data_request_manager.cc

@@ -110,7 +110,7 @@ void WebDataRequestManager::RequestCompleted(
   if (task_runner)
     task_runner->PostTask(FROM_HERE, std::move(task));
   else
-    base::PostTask(FROM_HERE, std::move(task));
+    base::ThreadPool::PostTask(FROM_HERE, std::move(task));
 }
 
 WebDataRequestManager::~WebDataRequestManager() {