Added addXSLStyleSheet and removeXSLStyleSheet to StyleEngine.

To apply XSLStyleSheet, we only need the first one. We don't need
to have all XSL stylesheets if the XML document has more than two
XSL stylesheets. So modified not to add XSLStyleSheet to
StyleSheetCandidateNodes. Instead, modified to make StyleEngine have
the first XSLStyleSheet.

BUG=367689
TEST=No new test, because this patch is just for refactoring.

Review URL: https://codereview.chromium.org/257363002

git-svn-id: svn://svn.chromium.org/blink/trunk@175775 bbb929c8-8fbe-4397-9dbb-9b2b20218538