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Make sure Blink WebRTCSessionDescriptionRequest and WebRTCVoidRequest are deleted on the correct thread in RTCPeerConnectionHandler.
Blink objects must always be destroyed on the main render thread. Since the object holding WebRTCSessionDescriptionRequest and WebRTCVoidRequest in rtc_peerconnection_handler.cc are refcounted, the destructor is not ensured to run on the main render thread.

BUG=470047

Review URL: https://codereview.chromium.org/1023443007

Cr-Commit-Position: refs/heads/master@{#321989}