[filesapp] Add global pointer-active handler

If the mouse/pointer/touch device is Pressed, adds a class to the root
HTML element (and remove it when the device is released), to allow you
to write CSS to prevent hover effects on inactive elements:

 html.pointer-active some-clickable-element:not(:active):hover {
   background-color: unset;
 }

Bug: 1062902
Change-Id: I6bafc00b608f69ea35d61a8a9e77f22fb597a56a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111810Reviewed-by: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751889}

ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js

@@ -429,8 +429,16 @@ class FileManagerUI {
     cr.ui.contextMenuHandler.setContextMenu(
         queryRequiredElement('.drive-welcome.page'), this.fileContextMenu);
 
-    // Add handlers.
+    // Add window resize handler.
     document.defaultView.addEventListener('resize', this.relayout.bind(this));
+
+    // Add global pointer-active handler.
+    const rootElement = document.documentElement;
+    ['pointerdown', 'pointerup'].forEach((eventType) => {
+      document.addEventListener(eventType, (e) => {
+        rootElement.classList.toggle('pointer-active', /down$/.test(e.type));
+      }, true);
+    });
   }
 
   /**