[directorytree] Re-locate splitter CSS in file_manager.css

file_manager.css is mostly visually ordered: the splitter comes after
the #directory-tree so move its CSS after the #directory-tree CSS.

Second, no need for separate margin-inline-{start,end} CSS rules when
the margin values are the same. Shorter to say "margin: 0 -3px" which
sets margin-inline-{start,end} to -3px anyway (aka it is RTL-aware).

Bug: 992819, 1002790
Change-Id: I12848afd496693aa0e122251a977a36708c5c5db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921010
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716055}

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

@@ -109,18 +109,6 @@ a:focus {
   flex-direction: column;
 }
 
-/* A vertical splitter between the directory tree and the file list. It is
-   actually a transparent area centered on the tree right border.*/
-div.splitter {
-  cursor: col-resize;
-  flex: none;
-  margin-inline-end: -3px;
-  margin-inline-start: -3px;
-  position: relative;
-  width: 6px;
-  z-index: 500;  /* Must be below the contextmenu (600). */
-}
-
 #directory-tree {
   bottom: 0;
   flex: none;
@@ -262,6 +250,17 @@ html[class*='col-resize'] #directory-tree .tree-row > .root-eject:hover {
   pointer-events: none;
 }
 
+/* A vertical splitter between the directory tree and the file list. It is
+   a transparent area centered on the directory tree right border.*/
+div.splitter {
+  cursor: col-resize;
+  flex: none;
+  margin: 0 -3px;
+  position: relative;
+  width: 6px;
+  z-index: 500;  /* Must be below the contextmenu (600). */
+}
+
 /* Breadcrumbs and things under the title but above the list view. */
 .dialog-header {
   align-items: center;