Add mock progress center items count helper for unittests

Add helper to count the number of items on the MockProgressCenter. Use
it in drive_sync_handler_unittest as an example, to remove a shed-load
of closure compiler complaints about the following code:

   Object.keys(progressCenter.items)

Move that code into the MockProgressCenter helper: getItemCount(), and
the Closure compiler will be a happy koala.

No change in test behavior, no new tests.

Bug: 907964
No-try: true
Change-Id: I3da5fda8226a64a0cad27dcb57b84fac6c7a43be
Reviewed-on: https://chromium-review.googlesource.com/c/1351201
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611038}

ui/file_manager/file_manager/background/js/drive_sync_handler_unittest.js

@@ -110,7 +110,7 @@ function testUniqueErrorIds() {
   });
 
   // Check that this created one item.
-  assertEquals(1, Object.keys(progressCenter.items).length);
+  assertEquals(1, progressCenter.getItemCount());
 
   // Dispatch another event.
   chrome.fileManagerPrivate.onDriveSyncError.listener_({
@@ -119,7 +119,7 @@ function testUniqueErrorIds() {
   });
 
   // Check that this created second item.
-  assertEquals(2, Object.keys(progressCenter.items).length);
+  assertEquals(2, progressCenter.getItemCount());
 }
 
 // Test that item IDs produced for quota errors are same.
@@ -131,7 +131,7 @@ function testErrorDedupe() {
   });
 
   // Check that this created one item.
-  assertEquals(1, Object.keys(progressCenter.items).length);
+  assertEquals(1, progressCenter.getItemCount());
 
   // Dispatch another event.
   chrome.fileManagerPrivate.onDriveSyncError.listener_({
@@ -140,7 +140,7 @@ function testErrorDedupe() {
   });
 
   // Check that this created second item.
-  assertEquals(1, Object.keys(progressCenter.items).length);
+  assertEquals(1, progressCenter.getItemCount());
 }
 
 // Test offline.
@@ -156,14 +156,14 @@ function testOffline() {
   });
 
   // Check that this created one item.
-  assertEquals(1, Object.keys(progressCenter.items).length);
+  assertEquals(1, progressCenter.getItemCount());
   assertEquals(
       ProgressItemState.PROGRESSING, progressCenter.items['drive-sync'].state);
   assertTrue(driveSyncHandler.syncing);
 
   chrome.fileManagerPrivate.onDriveConnectionStatusChanged.listener_();
 
-  assertEquals(1, Object.keys(progressCenter.items).length);
+  assertEquals(1, progressCenter.getItemCount());
   assertEquals(
       ProgressItemState.CANCELED, progressCenter.items['drive-sync'].state);
   assertFalse(driveSyncHandler.syncing);

ui/file_manager/file_manager/background/js/mock_progress_center.js

@@ -39,3 +39,13 @@ MockProgressCenter.prototype.requestCancel = function() {};
 MockProgressCenter.prototype.addPanel = function() {};
 
 MockProgressCenter.prototype.removePanel = function() {};
+
+/**
+ * Returns the number of unique keys in |this.items|.
+ * @return {number}
+ */
+MockProgressCenter.prototype.getItemCount = function() {
+  const array = Object.keys(
+      /** @type {!Object} */ (this.items));
+  return array.length;
+};