Bug 1545474 - Part 2: Modify eviction of content viewers to accomodate session history changes, r=peterv

When we need to evict content viewers, we group SHEntrySharedParentState,
corresponding to session history entries that need to be evicted, by their
content parent and send their id's to their corresponding parents for eviction.

Differential Revision: https://phabricator.services.mozilla.com/D32730

--HG--
extra : rebase_source : 17398d3ed2fdf5debba8224559c48f41b4cfbf2d
extra : source : 0feb286c5b85e6e2a0ac56058c596324ca28d817
extra : histedit_source : ba6c9eb5c6ce7013b80c407372eeab7146b07f51