Reorganize some code and add a test in hopes of getting better crash data

In the wild, we're seeing a crash in Document::completeURL that's hard to pin down,
in part because the line it's occurring is complicated. This patch picks apart that
complex line into a series of if statements, in the hope that future crash stacks
will have more useful data.

Also add a basic test for <link rel=stylesheet disabled>. The only existing test for
that feature involved the imperative API.

R=abarth@chromium.org
BUG=312410

Review URL: https://codereview.chromium.org/92833002

git-svn-id: svn://svn.chromium.org/blink/trunk@163014 bbb929c8-8fbe-4397-9dbb-9b2b20218538