Move early-return in TimeToSendPadding.
Move early-return in TimeToSendPadding.

Prevents taking send_critsect_ for checking sending status when not
actually intending to send padding.

BUG=
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1218093002

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9526}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 545727ecce444320328b825d65b287e844dca7cb

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