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We will be implementing vm shutdown in the UI, the most expedient way is
to have a means of telling the context_manager to reset its state. This
proves somewhat complicated, so we submit this refactoring initially
which does not change anything functional.

How this works:
 - we make the context a unique_ptr so that we can easily reset state by
   deleting it. This is easier than reinitializing it in place.
 - I removed several redundant fields of the context_manager_impl which
   were used to track state. The state of the manager is now implicit
   in the state of some of its members (particularly, the context and
   the task queue) rather than explicit in those variables.

Bug: b/172006764
Change-Id: I8f897f125beee6dd5eb55c784c310f133094c180
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507234
Commit-Queue: Nic Hollingum <hollingum@google.com>
Reviewed-by: Daniel Ng <danielng@google.com>
Cr-Commit-Position: refs/heads/master@{#823738}