Drop create flag from WebFileSystem::openFileSystem

The flag is always true now and no longer meaningful.
This change should land after corresponding chromium side change: http://crrev.com/26124003

Review URL: https://codereview.chromium.org/27029002

git-svn-id: svn://svn.chromium.org/blink/trunk@159531 bbb929c8-8fbe-4397-9dbb-9b2b20218538