Fix unexpected gray border on cr-dialog.

A recent CL (crrev.com/c/2258897) updates the behavior of the <dialog>
element. This adds CSS rules for the <dialog> element within cr-dialog
to remove a resulting gray border.

Fixed: 1148118
Change-Id: I6e6235f0202574374b184528810afe52b9f2d9d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538530Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#827545}

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.html

@@ -19,6 +19,8 @@
         box-shadow: 0 0 16px rgba(0, 0, 0, 0.12),
                     0 16px 16px rgba(0, 0, 0, 0.24);
         color: inherit;
+        max-height: initial;
+        max-width: initial;
         overflow-y: hidden;
         padding: 0;
         position: absolute;