Add a layout test to check result caching in FileReaderLoader::stringResult()

stringResult() returns cached result when m_rawData is not
modified since the last stringResult() call. Check it's working
correctly.

Review URL: https://chromiumcodereview.appspot.com/23465034

git-svn-id: svn://svn.chromium.org/blink/trunk@158364 bbb929c8-8fbe-4397-9dbb-9b2b20218538