Do not compound iframe margins when positioning

Instead of taking margin size into account twice when positioning
layers, just rely on the absolute position calculated during display
list construction.
Add support for local font faces. Improves fonts on rust lang and guide.
auto merge of #3676 : Ms2ger/servo/no-text-borrow, r=Manishearth

This should fix the most frequent intermittent wpt failure.
auto merge of #3673 : mrobinson/servo/iframe-margin, r=pcwalton

Instead of taking margin size into account twice when positioning
layers, just rely on the absolute position calculated during display
list construction.
Create an almost-correct form_owner() for convenience
Add form submission via input element
Add click handler for <input type=submit>
Filter out buttons which aren't the submitter while constructing the dataset
Update wpt expectations
Address review comments
auto merge of #3652 : Manishearth/servo/form-button, r=jdm

Partially fixes #3647
script: Use atom comparison in more places, especially for attributes.

75% improvement in style recalc for Guardians of the Galaxy.
auto merge of #3675 : glennw/servo/local-fonts, r=pcwalton
auto merge of #3623 : pcwalton/servo/use-atoms-2, r=jdm

75% improvement in style recalc for Guardians of the Galaxy.
embedding: implement cef_string_utf16_cmp()
embedding: begin cef string_list api

cef_string_list_alloc()
embedding: cef_string_list_size()
embedding: cef_string_list_append()
embedding: cef_string_list_value()
embedding: cef_string_list_clear()