Update web-platform-tests expectations.
Remove the unused 'obj' local from the construct hook.
Return slices from get_attribute_parts.
Fix comments in image_cache_task

The Cell that used to be here is now gone, and there is a typo in
another comment.
Fix comment about "impossible" render messages.

These messages are not impossible; they happen when a page contains iframes.
Comment spelling fix
Merge pull request #2837 from mbrubeck/comment-fix

Fix comment about "impossible" render messages. r=zwarich
Merge pull request #2836 from zwarich/image-cache-comment

Fix comments in image_cache_task; r=Ms2ger
Merge pull request #2830 from Ms2ger/constructor-unused-variable

Remove the unused 'obj' local from the construct hook; r=Manishearth
Merge pull request #2826 from Ms2ger/get_attribute_parts-slices

Return slices from get_attribute_parts; r=Manishearth
Remove an unnecessary string copy in the Text constructor.
Merge pull request #2838 from Ms2ger/text-clone

Remove an unnecessary string copy in the Text constructor.
Rename variables that refer to global objects.

This clarifies whether those variables will point to a Window object or an
arbitrary global object.

Note in particular that all IDL Constructors will accept an arbitrary global
object.
Simplify the implementation of cx_for_dom_reflector.
Put some 'use' statements in alphabetical order.
Make some DOM fields private.

Relevant to #2242.
Remove some Cells for immutable members.
Introduce abstractions for global scopes.

Part of #2811.
Merge pull request #2825 from Ms2ger/wpt-expectations

Update web-platform-tests expectations.
Support FormData outside Window scopes.