GitLab

 - change ProgressCenter externs file to be an @interface class
 - rename background page ProgressCenter to ProgressCenterImpl,
   and make it @implement the {ProgressCenter} interface
 - use ProgressCenterImpl as the implementation for Files App.
 - make MockProgressCenter @implement ProgressCenter for tests
 - add missing ProgressCenter methods to MockProgressCenter to
   make it Closure compile.
 - BUILD rules for progress_center, and mock_progress_center,
   were added in CL:1349139, and CL:1349147, respectively.

No change in test behavior, no new tests.

Bug: 908379
Change-Id: I087b72abce9221d464761724e3fcf2ccdb2dfad5
Reviewed-on: https://chromium-review.googlesource.com/c/1350563Reviewed-by: Stuart Langley <slangley@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611005}