layout: Make box building thread safe by pushing down styles into
non-element children during the cascade.

Fetching them from the parent isn't thread-safe.

Adds a test for anonymous block box inheritance.
auto merge of #1590 : pcwalton/servo/thread-safe-flow-construction, r=metajack

non-element children during the cascade.

Fetching them from the parent isn't thread-safe.

r? @metajack