styleForPage and PageRuleCollector should check whether rootElementSylte is available or not.

styleForPage can be invoked when document has no documentElement. So if document has no documentElement,
rootElementStyle is null, and accessing rootElementStyle causes crash.

BUG=288103
TEST=printing/print-document-without-documentElement-crash.html

Review URL: https://chromiumcodereview.appspot.com/23641007

git-svn-id: svn://svn.chromium.org/blink/trunk@157595 bbb929c8-8fbe-4397-9dbb-9b2b20218538