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
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