Add top-level and auxiliary browsing contexts to a group of BrowsingContexts on creation and store a pointer to that group in all children of the BrowsingContexts in the group. With this it is possible to compute the transitive closure of related browsing contexts. Since we'll not be using linked lists of BrowsingContexts for neither groups nor children we can move children to be an array of BrowsingContexts and adjust to use a the more convenient HashMap for roots.

In the end, after loads of investigations of whether test-linux64-asan/opt-web-platform-tests-e10s-5 was perma-failing or not, it now looks like it actually still only is intermittent, and that this patch hasn't made it start failing constantly. Will try to land asap. Full try here: https://treeherder.mozilla.org/#/jobs?repo=try&revision=eac92e73a0d92d7d0b96823b27bbede1db560fc3&group_state=expanded

Pushed by afarre@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/b068ba3cc5c3 Add BrowsingContextGroup. r=peterv