Introduce some (commented-out) tests for enumeration codegen.
Rewrite FindEnumStringIndex to distinguish 'value not found' and 'exception raised'.

This is in preparation for handling those cases differently.
Handle exceptions from FindEnumStringIndex correctly.

The previous code would return success from the JSNative with a pending
exception, potentially leading to assertion failures inside the JS engine
later.
Implement support for default values for enumeration arguments.
Add a few tests for dictionary codegen.
Support nullable strings in dictionaries.

We copy the string in all cases, so there's no need to handle the isMember
case differently.
Remove resource_task argument from css_link_listener.(fixes #2153)
auto merge of #2157 : lpy/servo/issue2153, r=Ms2ger

see #2153
auto merge of #2154 : Ms2ger/servo/enumerated-codegen, r=jdm
auto merge of #2155 : Ms2ger/servo/dictionary-codegen, r=jdm
Add web-platform-tests submodule and test runner
auto merge of #2089 : Manishearth/servo/wpt, r=Ms2ger

See #1479

This sets up a basic environment for running wpt. It instantiates a virtualenv, generates the manifest if it doesn't exist*, and runs the tests.

Currently @Ms2ger's [`run-in-servo` fork of wpt](https://github.com/Ms2ger/web-platform-tests/tree/run-in-servo/)  is used.

Note: I did everything in a shell script instead of make since one cannot run `source` from a makefile (the environment is immutable, apparently)


*It does not, however, update the manifest. We'll have to add an extra script for that.