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-Original-Commit-Position: refs/heads/master@{#19657}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2c7b7a633f6a6aef4467cc3b35a056a4dcc9d619
6 files changed