Adds new APIs Isolate::GetIncumbentContext() and
Context::BackupIncumbentScope to support
"the backup incumbent settings object stack" [1].

[1] https://html.spec.whatwg.org/multipage/webappapis.html#backup-incumbent-settings-object-stack

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I1ccea0e6fe2743fe5f3072b9e1236111ce2b1a42
Reviewed-on: https://chromium-review.googlesource.com/536728Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46246}