Flex the inside of the input control

We really need delegated focus from the container to the control, but this will
do for now.

TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/844763004

framework/sky-input.sky

@@ -15,6 +15,7 @@
     padding: 4px;
   }
   #control {
+    flex: 1;
     align-self: center;
     height: 1.2em;
     white-space: nowrap;