Add a DCHECK to RegisterModule to make sure it's called on the controller thread.
Add a DCHECK to RegisterModule to make sure it's called on the controller thread.

BUG=4508
TBR=perkj

Review URL: https://webrtc-codereview.appspot.com/43039004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8925}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bc4b93453c4935ee0857f80e82c437c6722df951

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