VideoCaptureImpl: Remove unused member variable |_capture_encoded_frame|
VideoCaptureImpl: Remove unused member variable |_capture_encoded_frame|

The end goal except cleanup is to remove webrtc::VideoFrame.

R=mflodman@webrtc.org, pbos@webrtc.org, perkj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37219004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8388}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 959dac749807806d9ba806b14a5698ef3853bd59

Alkalyne/webrtctrunk,krieger-od/webrtc,sippet/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,krieger-od/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,jchavanton/webrtc,jchavanton/webrtc,sippet/webrtc,sippet/webrtc,jchavanton/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,krieger-od/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,krieger-od/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk,sippet/webrtc,krieger-od/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,sippet/webrtc,Alkalyne/webrtctrunk,krieger-od/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk