Change destruction order to fix potential invalid pointer dereference.

BUG=657226

Review-Url: https://codereview.webrtc.org/2450953002
Cr-Original-Commit-Position: refs/heads/master@{#14792}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 54b0acb432b3cd4a20403862e2ed3ef33ae85c0f
1 file changed