Have StyleElement API take Document as a reference

Have StyleElement API take Document as a reference instead of a pointer
since the document cannot be NULL. This also avoid converting the
document reference into a pointer in call sites.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23847002

git-svn-id: svn://svn.chromium.org/blink/trunk@157042 bbb929c8-8fbe-4397-9dbb-9b2b20218538