cros OOBE login: Remove rounded corners for webview

Temp solution to get rid of flickering.

Fixed: 1016259
Change-Id: Ic8da146fa235537c64f31d3cfafa1e69752a3b73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1953106Reviewed-by: Roman Aleksandrov <raleksandrov@google.com>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722459}

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

@@ -37,8 +37,9 @@
 }
 
 #signin-frame {
-  border-top-left-radius: 4px;
-  border-top-right-radius: 4px;
+  /* TODO(https://crbug.com/1021866): Uncomment when the bug is fixed.
+   * border-top-left-radius: 4px;
+   * border-top-right-radius: 4px; */
   display: block;
   overflow: hidden;
   position: static;