Update MANIFEST.json to current web-platform-tests.

This was forgotten in the last update. The README is also updated as a
reminder for future updates.
Merge pull request #3169 from Ms2ger/update-manifest

Update MANIFEST.json to current web-platform-tests; r=jdm
Use macro getters for input elements
Introduce the NativePropertyHooks struct.

This will be required for the cross-origin wrapper work.
Merge pull request #3176 from Manishearth/more-macros

Use macro getters for input elements: r=Ms2ger
Merge pull request #3175 from Ms2ger/propertyhooks

Introduce the NativePropertyHooks struct; r=jdm
Introduce ConstantSpec::get_value.

I'll need it for cross-origin wrappers as well.
Merge pull request #3177 from Ms2ger/ConstantSpec

Introduce ConstantSpec::get_value; r=jdm