Fix progress bar issue when leaving NTP

When leaving the NTP, the progress bar of the toolbar was progressing
backward before starting the loading.
This CL fixes it.

Bug: 792791
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I49cbe4073b3d33d1e2fe45e44c588403c44dea7a
Reviewed-on: https://chromium-review.googlesource.com/813734Reviewed-by: Elodie Banel <lod@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522410}

ios/chrome/browser/ui/toolbar/clean/toolbar_view_controller.mm

@@ -571,8 +571,8 @@
   if (!loading) {
     [self stopProgressBar];
   } else {
-    [self.progressBar setProgress:0];
     [self.progressBar setHidden:NO animated:YES completion:nil];
+    [self.progressBar setProgress:0];
   }
 }