[directorytree] Make splitter inactive icon color G Grey 400

The splitter button contains a 6-dot icon that has G Grey 300 color in
files-ng design. Increase that to G Grey 400 to improve color contrast
against the files-ng files app white background color.

Bug: 1059691
Change-Id: I950f88856032150f2eab26414ec1bf1e90c903c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274124Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783800}

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

@@ -489,7 +489,7 @@ body.files-ng div.splitter .splitter-button .icon {
 }
 
 body.files-ng div.splitter:not(.splitter-active) .splitter-button .icon {
-  background-color: var(--google-grey-300);
+  background-color: var(--google-grey-400);
 }
 
 html:not(.col-resize) body.files-ng div.splitter:not(.splitter-active) .splitter-button:hover .icon {
@@ -497,7 +497,7 @@ html:not(.col-resize) body.files-ng div.splitter:not(.splitter-active) .splitter
 }
 
 html.pointer-active body.files-ng div.splitter:not(.splitter-active) .splitter-button:not(:active):hover .icon {
-  background-color: var(--google-grey-300);
+  background-color: var(--google-grey-400);
   cursor: default;
 }