[Chromoting] Set min-bitrate of video channel to 0

According to SendSideBandwidthEstimation,
https://cs.chromium.org/chromium/src/third_party/webrtc/modules/bitrate_controller/send_side_bandwidth_estimation.cc?rcl=033ea5f77140eef0c2defd4bd668de5dac9af3ea&l=392
the bitrate_bps won't be smaller than min_bitrate_configured_, i.e. the field
set by x-google-min-bitrate. This behavior is wrong or at least not what we have
expected in WebrtcTransport: when the bandwidth is fully used by video channel,
returning x-google-min-bitrate as the bitrate from OnTargetBitrateChanged() is
not expected.

I have verified this change on my machine, and it partially resolves the laggy
issue.

Bug: chromium:769391
Change-Id: I2c87a78af63b508b5f8b25b77e171c92a10c7426
Reviewed-on: https://chromium-review.googlesource.com/726312
Commit-Queue: Zijie He <zijiehe@chromium.org>
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509960}

remoting/protocol/webrtc_transport.cc

@@ -78,7 +78,7 @@ void UpdateCodecParameters(SdpMessage* sdp_message, bool incoming) {
   // underlying bug is fixed to handle high load at high bitrate.
   if (sdp_message->has_video()) {
     const char* kParameters =
-        "x-google-min-bitrate=1000; x-google-max-bitrate=20000";
+        "x-google-min-bitrate=0; x-google-max-bitrate=20000";
     bool param_added = sdp_message->AddCodecParameter("VP8", kParameters);
     param_added |= sdp_message->AddCodecParameter("VP9", kParameters);
     param_added |= sdp_message->AddCodecParameter("H264", kParameters);