[fsp] Enable thumbnails for provided file systems.

This patch allows to fetch thumbnails via metadata cache for provided file
systems.

TEST=Tested manually.
BUG=407954

Review URL: https://codereview.chromium.org/609263002

Cr-Commit-Position: refs/heads/master@{#297155}

ui/file_manager/file_manager/foreground/js/metadata/metadata_cache.js

@@ -793,8 +793,8 @@ ExternalProvider.prototype = {
  */
 ExternalProvider.prototype.supportsEntry = function(entry) {
   var locationInfo = this.volumeManager_.getLocationInfo(entry);
-  // TODO(mtomasz): Add support for provided file systems.
-  return locationInfo && locationInfo.isDriveBased;
+  return locationInfo && (locationInfo.isDriveBased ||
+      locationInfo.rootType === VolumeManagerCommon.RootType.PROVIDED);
 };
 
 /**