Add Progress Center's externs file to the BUILD and make it compile

Bug: 907962
Change-Id: Ia16246c0c4685b07b721e18e48d4c4f23bc210e6
Reviewed-on: https://chromium-review.googlesource.com/c/1349139Reviewed-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610534}

ui/file_manager/file_manager/background/js/BUILD.gn

@@ -81,6 +81,7 @@ js_library("closure_compile_externs") {
     "../../../externs/background/import_history.js",
     "../../../externs/background/import_runner.js",
     "../../../externs/background/media_scanner.js",
+    "../../../externs/background/progress_center.js",
     "../../../externs/background_window.js",
     "../../../externs/css_rule.js",
     "../../../externs/file_operation_progress_event.js",
@@ -314,7 +315,10 @@ js_library("progress_center") {
     "../../common/js:progress_center_common",
     "../../common/js:util",
     "../../foreground/js/ui:progress_center_panel",
+
   ]
+
+  externs_list = [ "//ui/file_manager/externs/background/progress_center.js" ]
 }
 
 js_library("task_queue") {

ui/file_manager/file_manager/background/js/progress_center.js

@@ -4,6 +4,10 @@
 
 /**
  * Progress center at the background page.
+ * Silence closure complaint about re-definition of the ProgressCenter here
+ * found when we began compiling ProgessCenter.
+ *
+ * @suppress {checkTypes}
  * @constructor
  * @struct
  */