Change Picture: Request ideal size for camera stream.

Request an ideal video stream size that matches the capture size.
Improves capture quality and makes sure we handle cropping
correctly.

Bug: 
Test: No "Video capture size too small" messages is UI log.
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I72ee55a4599e71bd249df4ff0d8eda89b73a785c
Reviewed-on: https://chromium-review.googlesource.com/719158
Commit-Queue: David Reveman <reveman@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508939}

ui/webui/resources/cr_elements/chromeos/cr_picture/cr_camera.js

@@ -132,7 +132,12 @@ Polymer({
       this.cameraStartInProgress_ = false;
     }.bind(this);
 
-    navigator.webkitGetUserMedia({video: true}, successCallback, errorCallback);
+    var videoConstraints = {
+      width: {ideal: CAPTURE_SIZE.width},
+      height: {ideal: CAPTURE_SIZE.height},
+    };
+    navigator.webkitGetUserMedia(
+        {video: videoConstraints}, successCallback, errorCallback);
   },
 
   /** Stops the camera stream capture if it's currently active. */