Move the computed style to a new SharedLayoutData struct.

Implementing the CSSOM requires giving the script task access to the
computed style for a node. Moving it into a new SharedLayoutData struct
member of LayoutDataRef seems to be the best way to achieve this.

This is the first step towards #1721.
auto merge of #2388 : zwarich/servo/cssom, r=jdm

Implementing the CSSOM requires giving the script task access to the
computed style for a node. Moving it into a new SharedLayoutData struct
member of LayoutDataRef seems to be the best way to achieve this.

This is the first step towards #1721.