Expand intervals of rescan operations when directory contents changed.

The listener of chrome.fileBrowserPrivate.onDirectoryChanged is called very frequently when many files are copied into a folder under Downloads.
In this case, we can't know the changed_files, so we need to rescan the folder.
As this operation is heavy, this patch adds some interval for each rescan.
Additionally, 1000ms delay might be too long for UI update, so I changed it to 500ms)

BUG=383232
TEST=manually tested the performance.

Review URL: https://codereview.chromium.org/460013003

Cr-Commit-Position: refs/heads/master@{#289871}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289871 0039d316-1c4b-4281-b951-d872f2087c98

ui/file_manager/file_manager/foreground/js/directory_model.js

@@ -6,7 +6,7 @@
 
 // If directory files changes too often, don't rescan directory more than once
 // per specified interval
-var SIMULTANEOUS_RESCAN_INTERVAL = 1000;
+var SIMULTANEOUS_RESCAN_INTERVAL = 500;
 // Used for operations that require almost instant rescan.
 var SHORT_RESCAN_INTERVAL = 100;
 
@@ -193,7 +193,9 @@ DirectoryModel.prototype.onWatcherDirectoryChanged_ = function(event) {
     }.bind(this));
   } else {
     // Invokes force refresh if the detailed information isn't provided.
-    this.rescanSoon(true);
+    // This can occur very frequently (e.g. when copying files into Downlaods)
+    // and rescan is heavy operation, so we keep some interval for each rescan.
+    this.rescanLater(true);
   }
 };