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}
diff --git a/webrtc/common_types.h b/webrtc/common_types.h
index 693d36d..059bd10 100644
--- a/webrtc/common_types.h
+++ b/webrtc/common_types.h
@@ -627,6 +627,7 @@
   bool                 frameDroppingOn;
   int                  keyFrameInterval;
   bool                 adaptiveQpMode;
+  bool                 automaticResizeOn;
   unsigned char        numberOfSpatialLayers;
   bool                 flexibleMode;
 };