When an iframe is created with display:none it sets the root layer to be
zero width and height. When updating the rect of the iframe from layout
send the entire rect rather than just the new origin, which handles the case
where the iframe has been made visible and now has a non-zero rect.
Instead of taking margin size into account twice when positioning
layers, just rely on the absolute position calculated during display
list construction.
Add iframe tests to their own subdirectory and add another test case
that used to trigger a fatal error. The new test case uses the
"allow-scripts" sandbox attribute to work around a script task failure
caused by the child frame sharing the same script task as the parent.
