Avoid reparsing an XSLT stylesheet after the first failure.

Certain libxslt versions appear to leave the doc in an invalid state when parsing fails. We should cache this result and avoid re-parsing.

(The test cannot be converted to text-only due to its invalid stylesheet).

R=inferno@chromium.org,abarth@chromium.org,pdr@chromium.org
BUG=271939

Review URL: https://chromiumcodereview.appspot.com/23103007

git-svn-id: svn://svn.chromium.org/blink/trunk@156248 bbb929c8-8fbe-4397-9dbb-9b2b20218538