Add voice over "container" behavior

This CL adds accessibility traits to the toolbars such as they are
acting as VoiceOver "container". If the user sets the accessibility
rotor to "containers", the user is able to go navigate quickly between
the toolbars and the webpage.

Bug: 856212
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I309a1308bb200bf9f90aaa4b164a3798c93c0f31
Reviewed-on: https://chromium-review.googlesource.com/1118275Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571111}

ios/chrome/browser/ui/toolbar/adaptive/adaptive_toolbar_view_controller.mm

@@ -69,6 +69,21 @@
   [super viewDidLoad];
   [self addStandardActionsForAllButtons];
 
+  if (@available(iOS 11.0, *)) {
+    [[NSNotificationCenter defaultCenter]
+        addObserver:self
+           selector:@selector(voiceOverChanged:)
+               name:UIAccessibilityVoiceOverStatusDidChangeNotification
+             object:nil];
+  } else {
+    [[NSNotificationCenter defaultCenter]
+        addObserver:self
+           selector:@selector(voiceOverChanged:)
+               name:UIAccessibilityVoiceOverStatusChanged
+             object:nil];
+  }
+  [self makeViewAccessibilityTraitsContainer];
+
   // Adds the layout guide to the buttons.
   self.view.toolsMenuButton.guideName = kToolsMenuGuide;
   self.view.tabGridButton.guideName = kTabSwitcherGuide;
@@ -224,6 +239,36 @@
   }
 }
 
+#pragma mark - Accessibility
+
+// Callback called when the voice over value is changed.
+- (void)voiceOverChanged:(NSNotification*)notification {
+  if (!UIAccessibilityIsVoiceOverRunning())
+    return;
+
+  __weak AdaptiveToolbarViewController* weakSelf = self;
+  dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)),
+                 dispatch_get_main_queue(), ^{
+                   // The accessibility traits of the UIToolbar is only
+                   // available after a certain amount of time after voice over
+                   // activation.
+                   [weakSelf makeViewAccessibilityTraitsContainer];
+                 });
+}
+
+// Updates the accessibility traits of the view to have it interpreted as a
+// container by voice over.
+- (void)makeViewAccessibilityTraitsContainer {
+  if (self.view.accessibilityTraits == UIAccessibilityTraitNone) {
+    // TODO(crbug.com/857475): Remove this workaround once it is possible to set
+    // elements as voice over container. For now, set the accessibility traits
+    // of the toolbar to the accessibility traits of a UIToolbar allows it to
+    // act as a voice over container.
+    UIToolbar* toolbar = [[UIToolbar alloc] init];
+    self.view.accessibilityTraits = toolbar.accessibilityTraits;
+  }
+}
+
 #pragma mark - Private
 
 // Updates all buttons visibility to match any recent WebState or SizeClass