Revert "Comment out DCHECK in dtor of VCMDecodedFrameCallback."

This reverts commit 9f016a0eb01db60c55dad640ddc03562d88cc087.

Reason for revert: Breaking internal project

Original change's description:
> Comment out DCHECK in dtor of VCMDecodedFrameCallback.
> Looking into the downstream issue now.
> 
> NoTry: true
> Tbr: ossu@webrtc.org
> Bug: webrtc:7361, webrtc:8907, chromium:695438
> Change-Id: Ib52b86cf26401c490b415b151916ec35f0716345
> Reviewed-on: https://webrtc-review.googlesource.com/56042
> Reviewed-by: Tommi <tommi@webrtc.org>
> Commit-Queue: Tommi <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22122}

TBR=ossu@webrtc.org,tommi@webrtc.org

Change-Id: I096205c1fe70131f6e1c866411f8838e12eafa92
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:7361, webrtc:8907, chromium:695438
Reviewed-on: https://webrtc-review.googlesource.com/56281
Reviewed-by: Lu Liu <lliuu@webrtc.org>
Commit-Queue: Lu Liu <lliuu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22142}
diff --git a/modules/video_coding/generic_decoder.cc b/modules/video_coding/generic_decoder.cc
index 5d22ffa..fe68998 100644
--- a/modules/video_coding/generic_decoder.cc
+++ b/modules/video_coding/generic_decoder.cc
@@ -34,8 +34,7 @@
 }
 
 VCMDecodedFrameCallback::~VCMDecodedFrameCallback() {
-  // TODO(tommi): Re-enable this DCHECK once downstream issues have been fixed.
-  // RTC_DCHECK(construction_thread_.CalledOnValidThread());
+  RTC_DCHECK(construction_thread_.CalledOnValidThread());
 }
 
 void VCMDecodedFrameCallback::SetUserReceiveCallback(