Move the directory tree section-start CSS rule to file_manager.css

Our tree.css file is derived from cr.ui.Tree tree.css file, and should
follow it rule-for-rule (mostly). The directory-tree section-start CSS
rule is specific to FilesApp: move it to file_manager.css.

Bug: 992819
Change-Id: I2fbfaaf539219da0f448fbeb1b6192b59b66c18d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1851627Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704974}

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

@@ -136,6 +136,10 @@ html[class*='col-resize'] #directory-tree {
   overflow-y: hidden;
 }
 
+#directory-tree .tree-item[section-start] {
+  border-top: 1px solid rgba(0, 0, 0, 0.10);
+}
+
 #directory-tree .tree-row {
   align-items: center;
   color: rgb(90, 90, 90);

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

@@ -36,10 +36,6 @@ tree {
       url(../images/files/ui/2x/expand_more_active.png) 2x);
 }
 
-div[section-start].tree-item {
-    border-top: 1px solid rgba(0, 0, 0, 0.10);
-}
-
 html[dir=rtl] .tree-row > .expand-icon {
   transform: rotate(90deg);
 }