Add some sanity CHECKs to webrtc::Call.
Add some sanity CHECKs to webrtc::Call.

These checks would help catching double-deletes, forgetting to destroy
streams and also catch if VideoEngine has held on to any stale
references.

BUG=1788
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/42929004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8866}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9b5f96e6a28ba7bf98a2b70b54ae14eb0a550156

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