ProcessingInstruction should parse attributes before invoking addStyleSheetCandidateNode

This patch makes it possible for us to see isXSLT in StyleEngine::addStyleSheetCandidateNode.

BUG=367689
TEST=just refactoring, no new tests

Review URL: https://codereview.chromium.org/253743002

git-svn-id: svn://svn.chromium.org/blink/trunk@172754 bbb929c8-8fbe-4397-9dbb-9b2b20218538