[iOS] Prevent FindBar from overlapping status bar

This CL adds a top constraints to the FindBar to avoid having it
overlapping the status bar when entering fullscreen.

Bug: 903802
Change-Id: I885c6eb8ca39a1810c1ce5c068d87ab01d68e22e
Reviewed-on: https://chromium-review.googlesource.com/c/1341912Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609290}

ios/chrome/browser/ui/find_bar/find_bar_controller_ios.mm

@@ -374,12 +374,22 @@ const NSTimeInterval kSearchShortDelay = 0.100;
   [parentView addSubview:self.view];
   self.view.accessibilityIdentifier = kFindInPageContainerViewId;
 
+  // Make sure that the Find in Page view bottom constraint has a lower priority
+  // than the constraint making sure that it doesn't overlap with the status
+  // bar.
+  NSLayoutConstraint* bottomConstraint =
+      [self.view.bottomAnchor constraintEqualToAnchor:toolbarView.bottomAnchor];
+  bottomConstraint.priority = UILayoutPriorityRequired - 1;
+
   [NSLayoutConstraint activateConstraints:@[
     [self.view.leadingAnchor constraintEqualToAnchor:parentView.leadingAnchor],
     [self.view.trailingAnchor
         constraintEqualToAnchor:parentView.trailingAnchor],
+    bottomConstraint,
+    [self.findBarView.topAnchor
+        constraintGreaterThanOrEqualToAnchor:self.view.safeAreaLayoutGuide
+                                                 .topAnchor],
     [self.view.topAnchor constraintEqualToAnchor:parentView.topAnchor],
-    [self.view.bottomAnchor constraintEqualToAnchor:toolbarView.bottomAnchor],
   ]];
 
   CGFloat duration = animated ? kAnimationDuration : 0;