Add support for marking a ref test as flaky on cpu and/or gpu.
Push get_buffer_requests_recursively into rust-layers

Also do some reorganization and abstraction in IOCompositor now that we
do not have to borrow self.scene.root.
Rename Compositor::ask_for_tiles to send_buffer_requests_for_all_layers

This name better reflects what the method does.
Remove CompositorData::add_child

The contents of this method have dwindled to two statements. Just
remove the method now.
Merge pull request #2976 from mrobinson/more-cleanup

Move get_buffer_requests into rust-layers from Servo and do some minor cleanup
Make scrolling tick by device pixels

Instead of converting a device offset into a page offset and then
converting back when positioning layers, keep scroll offsets in device
pixels. We still do conversions when calculating the scroll offset for
pinch zoom, but we can remove those when pinch zoom and contents zoom
are fully separated.
Merge pull request #2977 from mrobinson/scrolling

Make scrolling tick by device pixels
Merge pull request #2940 from glennw/reftest-conditions

Add support for marking a ref test as flaky on cpu and/or gpu.
Upgrade Rust.