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 - change the Crostini externs file to be an @interface class
   - add missing methods from the current implementation
   - add background BUILD rules to Closure compile {Crostini}
 - rename the background page Crostini to CrostiniImpl and
   make it @implements the {Crostini} class interface
 - use CrostiniImpl as the implementation for Files App: it is
   only instantiated on the Files app background page.
 - use CrostiniImpl for foreground unittests for now, until a
   mock (test-only) implementation is available.

No change in test behavior, no new tests.

Tbr: lucmult@
Bug: 911024
Change-Id: I580c9701374d6d8c1842a96040625c6be719ed64
Reviewed-on: https://chromium-review.googlesource.com/c/1358319
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613341}