Files.app: Add UI to toggle and clear item selection.

ui/file_manager/file_manager/foreground/js/ui/file_grid.js

Index: ui/file_manager/file_manager/foreground/js/ui/file_grid.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/file_grid.js b/ui/file_manager/file_manager/foreground/js/ui/file_grid.js
index fcad3f1cd5c8e585780fbd3575821e5219421e71..ad7aeabe68f246bdc62136c6cfde0144920271c6 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/file_grid.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/file_grid.js
@@ -189,6 +189,10 @@ FileGrid.decorateThumbnail = function(
 
   var checkmark = li.ownerDocument.createElement('div');
   checkmark.className = 'checkmark';
+  checkmark.addEventListener(
+      'mouseup', util.repeatMouseEventWithCtrlKey.bind(null, li));
+  checkmark.addEventListener(
+      'mousedown', util.repeatMouseEventWithCtrlKey.bind(null, li));
   frame.appendChild(checkmark);
 
   var bottom = li.ownerDocument.createElement('div');