[directorytree] Change file-ng active state tree-row background color

Original design required --google-blue-100. Later designs changed that
to --google-blue-50, so use that.

Bug: 992819
Change-Id: I938da4450cd85d8dc921b69f5cf0f919c3ef672b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032484Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737143}

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

@@ -318,7 +318,7 @@ html:not([class*='col-resize']) #directory-tree
 }
 
 #directory-tree .tree-row[active] > .file-row {
-  background-color: var(--google-blue-100);
+  background-color: var(--google-blue-50);
   color: var(--google-blue-600);
 }