Hide a download's progress indicator on completion to remove it from the a11y tree.

Bug: 773944
Change-Id: Ic1ed323b5facb6479bdbeaf319260269426400a8
Reviewed-on: https://chromium-review.googlesource.com/783250Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518378}

chrome/browser/ui/cocoa/download/md_download_item_view.mm

@@ -258,7 +258,7 @@ NSTextField* MakeLabel(
     imageView_.autoresizingMask =
         [NSView cr_localizedAutoresizingMask:NSViewMaxXMargin];
     ui::a11y_util::HideImageFromAccessibilityOrder(imageView_);
-    imageView_.alphaValue = 0;
+    imageView_.hidden = YES;
     [self addSubview:imageView_];
 
     base::scoped_nsobject<MDDownloadItemProgressIndicator> progressIndicator(
@@ -398,9 +398,9 @@ NSTextField* MakeLabel(
                    [NSAnimationContext
                        runAnimationGroup:^(NSAnimationContext* context) {
                          context.duration = 0.3;
-                         progressIndicator_.animator.alphaValue = 0;
+                         progressIndicator_.animator.hidden = YES;
                          context.duration = 1;
-                         imageView_.animator.alphaValue = 1;
+                         imageView_.animator.hidden = NO;
                        }
                        completionHandler:nil];
                  });