Remove all old layers when creating a new root.

Fixes #2259.
Check the pipeline ID of the new root layer.

This isn't related to any known issue, but is just to ensure that the IDs are
set correctly within this function.
Remove unused delete_layer code.
auto merge of #2575 : mbrubeck/servo/2259-remove-old-layers, r=pcwalton

This fixes #2259 which was caused by some of the previous layers sticking around incorrectly.  Depends on mozilla-servo/rust-layers#69.

Also includes some related cleanup as separate commits.

No reftest because our test harness doesn't yet support navigating to a new page (#2574).