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During shutdown this pointer might become dangling.
Instead, we fetch the value through |AssistantInteractionController::Get()|
whenever we need it (and check to ensure it is not nullptr).

I also checked if we need to protect the |IdentityManager*|, but that
should be safe as |BloomController| is created together with |Service|
(*), which also contains a pointer to |IdentityManager|.

(*) Here: http://shortn/_l71GCzVgX3

Bug: b/165356952
Tests: compiled
Change-Id: I9e1e9ed83db9a42221e1c2f0e0f3dbe14a33e0c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2372591
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803941}