Files app: Make sure to hide scrollbar on directory tree rows.

Depending on DPI settings, there is a chance to show unnecessary scroll bar on
directory tree rows.
As we don't need to scroll labels of directory tree rows, we can just overflow
property to hidden.

Bug: 787720
Test: Manually tested on sentry.
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I6735847daaa707244620e1039455848f4030ee45
Reviewed-on: https://chromium-review.googlesource.com/926024Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537742}

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

@@ -165,7 +165,7 @@ div.splitter {
   flex: auto;
   font-weight: 500;
   margin: 0 6px;
-  overflow-x: hidden;
+  overflow: hidden;
   text-overflow: ellipsis;
 }