Set files-toast z-index to 520.

Previously, selected files overlapped the files-toast; they have a

adjustable splitter between the directory tree and the files list was
also in front of the toast with a z-index: 500. The search autocomplete
suggestions have z-index: 550 and the toast should not overlap it so
pick a number in between: 520.

z-index: 2, but files-toast did not set one. Additionally, the
Change-Id: I307730c4287472963f0aa249dfce779f5202e4d3
Reviewed-on: https://chromium-review.googlesource.com/c/1350551Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610773}

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

@@ -2296,3 +2296,8 @@ cr-input:not(:defined) {
   position: absolute;
   z-index: 1000;  /* same height as files-tooltip */
 }
+
+files-toast {
+  /* Must be above the splitter (500) but below menu-like UI (550). */
+  z-index: 520;
+}