Add a note in <script> w/ invalid type loader LayoutTest about preloader

The current HTMLPreloadScanner preloads all <script src>s without checking
if they have valid type/languages.
LayoutTests/fast/dom/HTMLScriptElement/dont-load-unknown-type.html will
fail if HTML preload is enabled.

Currently HTML preload is skipped when HTMLParser is not blocked by a
script, so the test passes, but the test will fail once the preload
chunks are really sent to preloader.

BUG=329531

Review URL: https://codereview.chromium.org/697473003

git-svn-id: svn://svn.chromium.org/blink/trunk@184692 bbb929c8-8fbe-4397-9dbb-9b2b20218538