Use base::FilePath::BaseName() to get base name in Blink

rather than having platform-specific implementation in Blink code.

Chrome side patch: https://codereview.chromium.org/21733003/

BUG=267341
TEST=green bots

Review URL: https://chromiumcodereview.appspot.com/21733002

git-svn-id: svn://svn.chromium.org/blink/trunk@155522 bbb929c8-8fbe-4397-9dbb-9b2b20218538