Let VideoEncoderSoftwareFallbackWrapper own the wrapped encoder

Currently, ownership of the wrapped hardware encoder is handled outside
VideoEncoderSoftwareFallbackWrapper. It's easier if
VideoEncoderSoftwareFallbackWrapper owns and relases it instead.

BUG=webrtc:7925

Review-Url: https://codereview.webrtc.org/3007683002
Cr-Original-Commit-Position: refs/heads/master@{#19572}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f52d34d682a049028ef5f2d126721dce03a50d8b
5 files changed