[locationline] Do not show empty (aka root) paths

Searchbox activity can send requests to the location line breadcrumb to
display a root "/" path, when the current directory entry is a Recents,
a Fake entry, a mounted Zip file viz., all special directories that are
rooted according to the directory model API

   directoryModel.getCurrentDirEntry();

However, root paths have no path components and cause the location line
show() code to throw an exception accessing components[0].name, causing
files app to become totally unresponsive to user input.

Change location line to ignore requests to show a root (empty) path and
no JS exceptions is the result.

Bug: 1107391
Change-Id: I74523e94ec29f65342b393c2b176f8f4771e838d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308613Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790271}

ui/file_manager/file_manager/foreground/js/ui/location_line.js

@@ -36,7 +36,7 @@ class LocationLine extends cr.EventTarget {
   }
 
   /**
-   * Shows breadcrumbs. This operation is done without IO.
+   * Shows path of |entry|.
    *
    * @param {!Entry|!FakeEntry} entry Target entry or fake entry.
    */
@@ -47,6 +47,12 @@ class LocationLine extends cr.EventTarget {
 
     const components =
         PathComponent.computeComponentsFromEntry(entry, this.volumeManager_);
+
+    // Root "/" paths have no components, crbug.com/1107391.
+    if (!components.length) {
+      return;
+    }
+
     if (util.isFilesNg()) {
       this.updateNg_(components);
     } else {
@@ -55,8 +61,8 @@ class LocationLine extends cr.EventTarget {
   }
 
   /**
-   * Returns current path components built by the current directory entry.
-   * @return {!Array<!PathComponent>} Current path components.
+   * Returns the breadcrumb path components.
+   * @return {!Array<!PathComponent>}
    */
   getCurrentPathComponents() {
     return this.components_;
@@ -64,8 +70,7 @@ class LocationLine extends cr.EventTarget {
 
   /**
    * Updates the breadcrumb display for files-ng.
-   * @param {!Array<!PathComponent>} components Components to the
-   *     target path.
+   * @param {!Array<!PathComponent>} components Path components.
    * @private
    */
   updateNg_(components) {
@@ -79,12 +84,12 @@ class LocationLine extends cr.EventTarget {
       breadcrumbs.setSignalCallback(this.breadCrumbSignal_.bind(this));
     }
 
-    let crumbPath = components[0].name;
+    let path = components[0].name;
     for (let i = 1; i < components.length; i++) {
-      crumbPath += '/' + components[i].name;
+      path += '/' + components[i].name;
     }
-    breadcrumbs.path = crumbPath;
 
+    breadcrumbs.path = path;
     this.breadcrumbs_.hidden = false;
   }