[ios] Fix slimnav side swipe crash.

WebStateListObserving can pass a nil newWebState.

Bug: 935603
Change-Id: I1d7361199b2e1f238976c55bac98b7b0221740b6
Reviewed-on: https://chromium-review.googlesource.com/c/1489097
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635655}

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

@@ -596,6 +596,9 @@ const NSUInteger kIpadGreySwipeTabCount = 8;
     return;
   }
 
+  if (!webState)
+    return;
+
   // With slim nav enabled, disable SideSwipeController's edge swipe for a
   // typical navigation.  Continue to use SideSwipeController when on, before,
   // or after a native page.