Fix #1435: em units in font-size

Font-based units in the font-size property refer to the font-size
of the parent, not itself.
Implement Document's 'body' attribute

Implement Document's 'body' attribute (getter and setter). Proper
implementation of setter requires Node::ReplaceChild(), which is
currently a stub and will be done on a later step.

This patch is for:
https://github.com/mozilla/servo/issues/1428
auto merge of #1431 : brunoabinader/servo/document-body, r=jdm

Implement Document's 'body' attribute (getter and setter). Proper
implementation of setter requires Node::ReplaceChild(), which is
currently a stub and will be done on a later step.

This patch is for:
https://github.com/mozilla/servo/issues/1428
Add a test for percentages in font-size
auto merge of #1436 : SimonSapin/servo/font-size-em, r=kmcallister

Font-based units in the font-size property refer to the font-size of the parent, not itself.