Use new operator consistently for constructor-like functions.

Function MainPanelScrollBar() is not a constructor, but it behaves like a constructor.
We can use new consistently for such functions.

BUG=none
TEST=manually

Review URL: https://codereview.chromium.org/484083002

Cr-Commit-Position: refs/heads/master@{#290741}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290741 0039d316-1c4b-4281-b951-d872f2087c98

ui/file_manager/file_manager/foreground/js/directory_tree.js

@@ -987,7 +987,7 @@ DirectoryTree.prototype.decorate = function(
   chrome.fileBrowserPrivate.onDirectoryChanged.addListener(
       this.privateOnDirectoryChangedBound_);
 
-  this.scrollBar_ = MainPanelScrollBar();
+  this.scrollBar_ = new MainPanelScrollBar();
   this.scrollBar_.initialize(this.parentNode, this);
 
   /**

ui/file_manager/file_manager/foreground/js/file_table.js

@@ -298,7 +298,7 @@ FileTable.decorate = function(self, metadataCache, volumeManager, fullPage) {
   self.setRenderFunction(self.renderTableRow_.bind(self,
       self.getRenderFunction()));
 
-  self.scrollBar_ = MainPanelScrollBar();
+  self.scrollBar_ = new MainPanelScrollBar();
   self.scrollBar_.initialize(self, self.list);
   // Keep focus on the file list when clicking on the header.
   self.header.addEventListener('mousedown', function(e) {