Revert "Make Document a Node." for breaking Acid1

This reverts commit 4e47d59165d186d0938fe9ffd726b2c1b83d50f4.

Conflicts:

	src/components/script/dom/domparser.rs
auto merge of #1128 : pcwalton/servo/revert-document-node, r=metajack

This reverts commit 4e47d59165d186d0938fe9ffd726b2c1b83d50f4.

Conflicts:

	src/components/script/dom/domparser.rs

r? @metajack
auto merge of #1130 : kmcallister/servo/contenttest, r=metajack

**This build is expected to fail** at first, because `test_img_width_height` is broken on `master`. It's the simplest way I know to check that with this change, buildbot will indeed catch failing content tests.

After that happens I'll push the commits to fix `test_img_width_height` and request a re-review.
test_img_width_height: Reformat code
test_img_width_height: Give layout a chance to run after setting size

Workaround for #1129.  When that's fixed, revert this commit so we don't
regress.