Update omnibox clear button visibility on focus.

Monitor omnibox focus notification to update the clear button visibility
on focus.

Bug: 866446, 865019, 865013, 864511
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Iecf54d4550e4c6fe3f3988a54602c4d32d417f89
Reviewed-on: https://chromium-review.googlesource.com/1143860
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577161}

ios/chrome/browser/ui/omnibox/omnibox_view_controller.mm

@@ -85,6 +85,13 @@ const CGFloat kClearButtonSize = 28.0f;
   self.textField.placeholderTextColor = [self placeholderAndClearButtonColor];
   self.textField.placeholder = l10n_util::GetNSString(IDS_OMNIBOX_EMPTY_HINT);
   [self setupClearButton];
+
+  // TODO(crbug.com/866446): Use UITextFieldDelegate instead.
+  [[NSNotificationCenter defaultCenter]
+      addObserver:self
+         selector:@selector(textFieldDidBeginEditing)
+             name:UITextFieldTextDidBeginEditingNotification
+           object:self.textField];
 }
 
 - (void)traitCollectionDidChange:(UITraitCollection*)previousTraitCollection {
@@ -122,6 +129,14 @@ const CGFloat kClearButtonSize = 28.0f;
                         : [UIColor colorWithWhite:0 alpha:0.3];
 }
 
+#pragma mark notification callbacks
+
+// Called on UITextFieldTextDidBeginEditingNotification for self.textField.
+- (void)textFieldDidBeginEditing {
+  // Update the clear button state.
+  [self updateClearButtonVisibility];
+}
+
 #pragma mark clear button
 
 // Omnibox uses a custom clear button. It has a custom tint and image, but