Files app: Stop horizontal scroll bar for file header

Avoid dialog-header expanding its width, when the width expands it
causes the parent container to be wider causing a horizontal scroll that
makes the directory tree to be hidden and the file list to be wider and
potentially having a horizontal scroll which isn't desired.

The "overflow-y: hidden" will cause the content of the toolbar
(breadcrumbs and buttons) to be out of the view port if the toolbar is
too narrow, but using Tab key can focus these elements and bring them to
the view port.

This is a follow up on CL:2098140 which fixes the size of file list.

Bug: 1059643
Change-Id: I9ab3410b978e942437c2d65aac460e7efcbd0bc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098149Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749529}

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

@@ -486,6 +486,7 @@ body.files-ng div.splitter.splitter-active .splitter-button .icon {
   border-bottom: 1px solid var(--cr-separator-color, rgb(224, 224, 224));
   color: var(--google-grey-700);
   height: 56px;
+  overflow-x: hidden;
 }
 
 body.check-select .dialog-header {