DevTools: Factor out WebInspector.FileBasedSearchResultsPane.FileTreeElement

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/223213006

git-svn-id: svn://svn.chromium.org/blink/trunk@170774 bbb929c8-8fbe-4397-9dbb-9b2b20218538