Remove thread check in WebTaskRunner

The DCHECK in WebTaskRunner::PostTask used to ensure that a same thread
WTF::Function is posted to the original thread. However, after it is
merged to base::OnceCallback, it wrongly rejects base::OnceCallback
which may be passed through base::SingleThreadTaskRunner::PostTask.

This CL removes the DCHECK to prevent the potential failure. The
restriction itself is still checked by a callback wrapper made by
WTF::Bind.

Change-Id: I5b0cf0a6beb3b88a66a1ce89a8df466379cb9c74
Reviewed-on: https://chromium-review.googlesource.com/813574
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522441}

third_party/WebKit/Source/platform/WebTaskRunner.cpp

@@ -132,7 +132,6 @@ void WebTaskRunner::PostDelayedTask(const WebTraceLocation& location,
 
 void WebTaskRunner::PostTask(const WebTraceLocation& location,
                              WTF::Closure task) {
-  DCHECK(RunsTasksInCurrentSequence());
   PostDelayedTask(location, std::move(task), base::TimeDelta());
 }