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lacros-chrome is allowed to make synchronous mojo IPC calls to
ash-chrome. To avoid deadlocks, the sequence responsible for handling
lacros-chrome mojo IPC must never block. Prior to this CL, the class
LacrosChromeServiceImpl was affine to the main-thread, which could and
did block. See bug for a concrete example of a deadlock.

This CL create a new private class
LacrosChromeServiceNeverBlockingState. This class is affine to a
never-blocking sequence. Most state has been moved from
LacrosChromeServiceImpl to the private class. The exception is the public
endpoint mojo::Remote<crosapi::mojom::SelectFile>, which is affine to
the main thread.

Bug: 1103765
Change-Id: I462311f7b2b68db5eeaab799a83d2bb6391dc575
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293011
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788810}