Revert of Add _decoder CHECK to VCMGenericDecoder constructor. (patchset #2 id:20001 of https://codereview.webrtc.org/1485713002/ )

Reason for revert:
Speculative revert since a downstream test started failing with this.

Original issue's description:
> Add _decoder CHECK to VCMGenericDecoder constructor.
>
> This should never be using a null decoder, but it looks like it's
> crashing out in the field. Adding a CHECK to see if it catches any
> interesting stack traces.
>
> Also making the _decoder pointer const to show that it should never be
> changing.
>
> BUG=chromium:563299
> R=stefan@webrtc.org
>
> Committed: https://chromium.googlesource.com/external/webrtc/+/a443ec1a75d10be0f24fa0163f23e1555d4c53cf

TBR=stefan@webrtc.org,pbos@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:563299

Review URL: https://codereview.webrtc.org/1490703002

Cr-Commit-Position: refs/heads/master@{#10851}
diff --git a/webrtc/modules/video_coding/generic_decoder.h b/webrtc/modules/video_coding/generic_decoder.h
index 1375996..d79b5e5 100644
--- a/webrtc/modules/video_coding/generic_decoder.h
+++ b/webrtc/modules/video_coding/generic_decoder.h
@@ -65,7 +65,7 @@
 {
     friend class VCMCodecDataBase;
 public:
-    VCMGenericDecoder(VideoDecoder* decoder, bool external);
+    VCMGenericDecoder(VideoDecoder& decoder, bool isExternal = false);
     ~VCMGenericDecoder();
 
     /**
@@ -102,7 +102,7 @@
     VCMDecodedFrameCallback*    _callback;
     VCMFrameInformation         _frameInfos[kDecoderFrameMemoryLength];
     uint32_t                    _nextFrameInfoIdx;
-    VideoDecoder* const         _decoder;
+    VideoDecoder&               _decoder;
     VideoCodecType              _codecType;
     bool                        _isExternal;
     bool                        _keyFrameDecoded;