Do not forward declare WebRtcVoiceChannelInfo.

Clang / libc++ does not support incomplete types in
templates. See:
http://clang.llvm.org/compatibility.html#undep_incomplete

This fixes the fixes the "error: field has incomplete type" compiler error.

This patch will be squashed into the next chromium update.

Change-Id: I0af464b517d112a1a88a4598105eb3d60a91c723
Reviewed-by: Andras Becsi <andras.becsi@digia.com>