Improve ChromeOS OOBE app-downloading screen

This commit adapts chromeOS OOBE 'app-downloading' screen for devices
with small displays.

Tested locally.

Bug: 1002096
Change-Id: I6fc689c6573bb0370d4465fc45ed796efdcd1199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1844971
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703942}

chrome/browser/resources/chromeos/login/app_downloading.css

@@ -6,8 +6,6 @@
   color: #333;
   font: 13px Roboto, sans-serif;
   font-weight: 400; /* roboto-regular */
-  height: 640px;
-  width: 768px;
 }
 
 #app-downloading-video-container {