Implement [LenientThis] support and enable it in Document.webidl

Closes issue #3760
Add content test for [LenientThis] attribute Document.onreadystatechange
Disable Document interface attribute onreadystatechange test

idlharness.js does not understand [LenientThis] yet
auto merge of #3812 : mukilan/servo/lenient-this, r=Ms2ger

Closes issue #3760
Cleanup some code in image_cache_task.rs.
auto merge of #3815 : Ms2ger/servo/image-cache-task, r=glennw
Fix cargo.lock that got out of sync when two successive commits (1) updated the SHA (2) added the dependency to util with the old SHA.
auto merge of #3816 : glennw/servo/cargo-lock-fix, r=Manishearth
Fix a script -> layout race

When content changed, script used to immediately dirty the changed
nodes regardless of what layout was doing at the same time. The
correct thing to do in that situation is to queue the nodes, then
dirty them after layout is finished and re-run layout.

This patch makes that change.

There's also some extra debugging goodness in this patch that I found
useful when tracking down the bug.

r? @jdm @pcwalton