Remove initial from directory-changed event.
The flag is only used for disabling auto-window-closing when a device is
removed. Instead, assign null to previousDirEntry for initial directory change.
BUG=268817
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*".
R=mtomasz@chromium.org
Review URL: https://codereview.chromium.org/
23875049
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225136 0039d316-1c4b-4281-b951-d872f2087c98