Fix bug of VoiceOver does not read out EmptyStateView text in the Tab Grid

The TabGridEmptyStateView is ignored in Voiceover, and changing the constraints
of the view can fix it. There is still a bug that happens when user open the
Voiceover in regular TabGrid for the first time, but that bugs exists before this
CL and will be fixed later.

Bug: 876873
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ifb07363b0e216bbb7b0ee4e3c05d7bfd59d4e415
Reviewed-on: https://chromium-review.googlesource.com/1202146Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588853}

ios/chrome/browser/ui/tab_grid/tab_grid_empty_state_view.mm

@@ -76,16 +76,15 @@
   bottomLabel.textAlignment = NSTextAlignmentCenter;
   [self addSubview:bottomLabel];
   [NSLayoutConstraint activateConstraints:@[
+    [topLabel.topAnchor constraintEqualToAnchor:self.topAnchor],
     [topLabel.leadingAnchor constraintEqualToAnchor:self.leadingAnchor],
     [topLabel.trailingAnchor constraintEqualToAnchor:self.trailingAnchor],
     [topLabel.bottomAnchor
-        constraintEqualToAnchor:self.centerYAnchor
-                       constant:-kTabGridEmptyStateVerticalMargin / 2.0f],
-    [bottomLabel.topAnchor
-        constraintEqualToAnchor:self.centerYAnchor
-                       constant:kTabGridEmptyStateVerticalMargin / 2.0f],
+        constraintEqualToAnchor:bottomLabel.topAnchor
+                       constant:-kTabGridEmptyStateVerticalMargin],
     [bottomLabel.leadingAnchor constraintEqualToAnchor:self.leadingAnchor],
     [bottomLabel.trailingAnchor constraintEqualToAnchor:self.trailingAnchor],
+    [bottomLabel.bottomAnchor constraintEqualToAnchor:self.bottomAnchor],
   ]];
 }