Add tests to read a directory with many entires

BUG=347900
TEST=run_webkit_tests.sh fast/filesystem/read-directory-many.html
TEST=run_webkit_tests.sh fast/filesystem/workers/read-directory-sync-many.html

Review URL: https://codereview.chromium.org/190653002

git-svn-id: svn://svn.chromium.org/blink/trunk@168906 bbb929c8-8fbe-4397-9dbb-9b2b20218538