VP9: Add automaticeResize to codec setting.

Added unittest.
This setting allows for dynamic resizing at low bitrates.
Setting is off by default for now.

BUG=
R=mflodman@webrtc.org, stefan@webrtc.org
TBR=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1323943007 .

Cr-Commit-Position: refs/heads/master@{#9976}

webrtc/modules/video_coding/main/source/codec_database.cc

@@ -61,6 +61,7 @@ VideoCodecVP9 VideoEncoder::GetDefaultVp9Settings() {
   vp9_settings.frameDroppingOn = true;
   vp9_settings.keyFrameInterval = 3000;
   vp9_settings.adaptiveQpMode = true;
+  vp9_settings.automaticResizeOn = false;
   vp9_settings.numberOfSpatialLayers = 1;
   vp9_settings.flexibleMode = false;
   return vp9_settings;