[Video] Remove wrong DCHECK from EnableVideoTrackIfNeeded

The DCHECK doesn't make sense as we might reenable the track during
suspend/resume while in the background.

BUG=683152
TEST=none

Review-Url: https://codereview.chromium.org/2641373002
Cr-Commit-Position: refs/heads/master@{#445128}
(cherry picked from commit fd01a87)

Review-Url: https://codereview.chromium.org/2651533007 .
Cr-Commit-Position: refs/branch-heads/2987@{#58}
Cr-Branched-From: ad51088-refs/heads/master@{#444943}

media/blink/webmediaplayer_impl.cc

@@ -2181,8 +2181,6 @@ bool WebMediaPlayerImpl::IsBackgroundOptimizationCandidate() const {
 }
 
 void WebMediaPlayerImpl::EnableVideoTrackIfNeeded() {
-  DCHECK(!IsHidden());
-
   // Don't change video track while the pipeline is resuming or seeking.
   if (is_pipeline_resuming_ || seeking_)
     return;