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