Delete static method TaskQueue::IsCurrent.

The static method takes a queue name as argument. Used only in one
place, a DCHECK in IncomingVideoStream, which is converted to use
the non-static version of IsCurrent.

BUG=webrtc:8166

Review-Url: https://codereview.webrtc.org/3004873002
Cr-Commit-Position: refs/heads/master@{#19657}
diff --git a/webrtc/common_video/incoming_video_stream.cc b/webrtc/common_video/incoming_video_stream.cc
index be16cfb..b1c3191 100644
--- a/webrtc/common_video/incoming_video_stream.cc
+++ b/webrtc/common_video/incoming_video_stream.cc
@@ -37,7 +37,7 @@
 
  private:
   bool Run() override {
-    RTC_DCHECK(rtc::TaskQueue::IsCurrent(kIncomingQueueName));
+    RTC_DCHECK(stream_->incoming_render_queue_.IsCurrent());
     if (stream_->render_buffers_.AddFrame(std::move(frame_)) == 1)
       stream_->Dequeue();
     return true;