Change Dialog background color only when it's visible

This fixes the perception to users that the dialog is taking too long
to disappear.

Test: No change in behavior, so current test suite should cover it. Additionally, manually tested file manager file deletion confirmation.
Bug: 883659
Change-Id: I414a7282f70a64c0826954ef850a16e0bc37182e
Reviewed-on: https://chromium-review.googlesource.com/1242725Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595703}

ui/file_manager/file_manager/foreground/css/common.css

@@ -193,9 +193,11 @@ html[dir='rtl'] .entry-name {
 }
 
 /* Pop-up dialogs. */
+.cr-dialog-container.shown {
+  background-color: rgba(0, 0, 0, 0.6);
+}
 
 .cr-dialog-container {
-  background-color: rgba(0, 0, 0, 0.6);
   display: flex;
   height: 100%;
   left: 0;