Cleanup Document.find_fragment_node.
Fix up and run unit tests.

This adds the subpackages to `./mach test-unit`.
Use or_else in Document.find_fragment_node.
auto merge of #3562 : Ms2ger/servo/cleanup-fragment-node, r=jdm
auto merge of #3566 : metajack/servo/fixup-unit-tests, r=mbrubeck

This adds the subpackages to `./mach test-unit`.
Add Android build config to mach/servobuild
auto merge of #3568 : mbrubeck/servo/mach-android, r=metajack

This allows setting Android toolchain paths in `.servobuild`, and also adds a `--android` option that sets the correct default target and causes `mach build` to also build the APK.
Account for RenderLayer position when optimizing display list

The page_rect passed to DisplayListOptimizer is relative to the
RenderLayer origin, but the display list components are relative to the
page origin. Before passing the page rect to the display list, we
translate it by the RenderLayer position.
auto merge of #3567 : mrobinson/servo/optimizer, r=zwarich

The page_rect passed to DisplayListOptimizer is relative to the
RenderLayer origin, but the display list components are relative to the
page origin. Before passing the page rect to the display list, we
translate it by the RenderLayer position.
Replace Cell<Option<JS<T>>> with MutNullableJS<T>

https://github.com/servo/servo/issues/3564
auto merge of #3569 : andrewguertin/servo/mutnullablejs, r=Manishearth

https://github.com/servo/servo/issues/3564
Stop reference-counting the ScriptTask.

The ScriptTask is owned by the proc in ScriptTaskFactory::create.

We started using a managed box for what would eventually become ScriptTask in
d21d27e08633b6a7f2b774a28c65c81d1eedbeaf.
Cleanup URL handling in ScriptTask::load a bit.

This should make it clearer that we always pass a URL to Document::new, and
avoids an unnecessary unwrap() call.
Remove the managed_boxes feature.
Remove Traceable from attr.rs
Remove Traceable from browsercontext.rs
Remove Traceable from characterdata.rs
Remove Traceable from customevent.rs
Remove Untraceable from dedicatedglobalworkerscope.rs
Remove Untraceable from canvasrenderingcontext2d.rs