[iOS] Do not scroll to top when in fullscreen

This CL disable the "scroll to top" action when tapping the status bar
when fullscreen is enabled.

Bug: 876716
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: If2a199be39da0de596e8d945e9fc4c42c65ad009
Reviewed-on: https://chromium-review.googlesource.com/1185098Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585766}

ios/chrome/browser/ui/fullscreen/fullscreen_web_view_proxy_observer.mm

@@ -57,10 +57,14 @@
 
 - (BOOL)webViewScrollViewShouldScrollToTop:
     (CRWWebViewScrollViewProxy*)webViewScrollViewProxy {
-  // Inform FullscreenUIElements that the content is going to be scrolled to the
-  // top.
+  if (self.model->progress() > 0.05) {
+    // Inform FullscreenUIElements that the content is going to be scrolled to
+    // the top.
     self.mediator->ScrollToTop();
     return YES;
+  } else {
+    return NO;
+  }
 }
 
 @end