[Managed Session] Housekeeping on Managed Session platform delegates

Continuation of work from crrev.com/c/1202147, fixing up how delegates
are set and persisted (using the base::NoDestructor pattern).

Bug: 865947
Change-Id: Ib8768fe71e91c5f1bad400ec047b2c941a939608
Reviewed-on: https://chromium-review.googlesource.com/1210502
Reviewed-by: Xiyuan Xia <[email protected]>
Reviewed-by: Devlin <[email protected]>
Commit-Queue: Ivan Ĺ andrk <[email protected]>
Cr-Commit-Position: refs/heads/master@{#589940}
11 files changed