Delete capturers after destroying streams in test.
Delete capturers after destroying streams in test.

Since the renderers in CallTest.SendsAndReceiveStreams also stopped the
capturers they must be deleted after the VideoReceiveStream is stopped
or an use-after-free may occur.

R=mflodman@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/5969004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 919f87fb36af15bd995eca00c53a1dee1c8b5000

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc