Remove some code duplication in the Compositor

It is possible to share the code which creates root layers.
auto merge of #3964 : mrobinson/servo/code-duplication, r=larsbergstrom

It is possible to share the code which creates root layers.
Update web-platform-tests.
cef: Don't link against librustc in the final build artifact
auto merge of #4040 : pcwalton/servo/librustc-linkage, r=metajack

r? @zmike
auto merge of #4039 : Ms2ger/servo/wpt_20141119, r=jdm