Hide directory tree scrollbar when splitter resize is active

When the user is resizing the app using the splitter, and the tree has
has overflow-y content, a scroll bar appears. That is annoying. Fix by
hiding directory scroll bars while the splitter is active.

Bug: 1002790
Change-Id: Ie03c75f020a9f0cf573b7c46bf0d204190e68d66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1851789Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704550}

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

@@ -109,8 +109,8 @@ a:focus {
   flex-direction: column;
 }
 
-/* A vertical splitter between the roots list and the file list. It is actually
-   a transparent area centered on the roots list right border.*/
+/* 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;
@@ -132,6 +132,10 @@ div.splitter {
   top: 0;
 }
 
+html[class*='col-resize'] #directory-tree {
+  overflow-y: hidden;
+}
+
 #directory-tree .tree-row {
   align-items: center;
   color: rgb(90, 90, 90);