Fix Download Manager UI presentation during side swipe tab switching.

crbug.com/819416 was happening, because WebStateObserver::WasShown was
called before the animation was completed. This CL moves actual tab
switch operation to animation completion block to fix WasShown timing.
However it is necessary to send WebStateObserver::WasHidden before
animation is started, so this cl adds WebState::WasHidden call if the
tab was actually changed.

Bug: 819416
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ic45ad02e76674a4bd861fe2626f7d3c32aea3a11
Reviewed-on: https://chromium-review.googlesource.com/953075Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541533}

ios/chrome/browser/ui/side_swipe/card_side_swipe_view.mm

@@ -438,9 +438,11 @@ const CGFloat kResizeFactor = 4;
     }
   }
 
-  // Changing the model even when the tab is the same at the end of the
-  // animation allows the UI to recover.
-  [model_ setCurrentTab:destinationTab];
+  if (destinationTab != model_.currentTab) {
+    // The old tab is now hidden. The new tab will be inserted once the
+    // animation is complete.
+    model_.currentTab.webState->WasHidden();
+  }
 
   // Make sure the dominant card animates on top.
   [dominantCard.superview bringSubviewToFront:dominantCard];
@@ -453,6 +455,9 @@ const CGFloat kResizeFactor = 4;
         [_rightCard setFrame:rightFrame];
       }
       completion:^(BOOL finished) {
+        // Changing the model even when the tab is the same at the end of the
+        // animation allows the UI to recover.
+        [model_ setCurrentTab:destinationTab];
         [_leftCard setImage:nil];
         [_rightCard setImage:nil];
         [_leftCard setTopToolbarImage:nil isNewTabPage:NO];