Fix a bug that generic thumbnail is visible behind transparent thumbnail.

BUG=459326
TEST=open a folder which contains transparent image and confirm that generic thumbnail is not visible behind thumbnail.

Review URL: https://codereview.chromium.org/935333003

Cr-Commit-Position: refs/heads/master@{#317033}

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

@@ -1053,6 +1053,7 @@ body.selecting .thumbnail-grid .thumbnail-item[selected] .checkmark {
 
 .thumbnail-grid .img-container > .thumbnail {
   -webkit-user-drag: none;
+  background-color: rgb(230, 230, 230);
   background-position: center;
   background-size: cover;
   height: 100%;