[iOS] Rename OmniboxTextfieldIOS' clearButton method.

UITextField appears to use an internal, undocumented and unprefixed -clearButton
method.  This conflicts with our -clearButton method causing unexpected
behavior.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I54434cd143bef76975f7aea1ffe811dc9135eed1
Reviewed-on: https://chromium-review.googlesource.com/962442
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549535}

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

@@ -298,26 +298,26 @@ NSString* const kOmniboxFadeAnimationKey = @"OmniboxFadeAnimation";
                 completionAnimator:(UIViewPropertyAnimator*)completionAnimator {
   // Hide the rightView button so its not visibile on its initial layout
   // while the expan animation is happening.
-  self.clearButton.hidden = YES;
-  self.clearButton.alpha = 0;
-  self.clearButton.frame =
+  self.clearButtonView.hidden = YES;
+  self.clearButtonView.alpha = 0;
+  self.clearButtonView.frame =
       CGRectLayoutOffset([self rightViewRectForBounds:self.bounds],
                          [self clearButtonAnimationOffset]);
 
   [completionAnimator addAnimations:^{
-    self.clearButton.hidden = NO;
-    self.clearButton.alpha = 1.0;
+    self.clearButtonView.hidden = NO;
+    self.clearButtonView.alpha = 1.0;
 
-    self.clearButton.frame = CGRectLayoutOffset(
-        self.clearButton.frame, -[self clearButtonAnimationOffset]);
+    self.clearButtonView.frame = CGRectLayoutOffset(
+        self.clearButtonView.frame, -[self clearButtonAnimationOffset]);
   }];
 }
 
 - (void)addContractOmniboxAnimations:(UIViewPropertyAnimator*)animator {
   [animator addAnimations:^{
-    self.clearButton.alpha = 0;
-    self.clearButton.frame = CGRectLayoutOffset(self.clearButton.frame,
-                                                kToolbarButtonAnimationOffset);
+    self.clearButtonView.alpha = 0;
+    self.clearButtonView.frame = CGRectLayoutOffset(
+        self.clearButtonView.frame, kToolbarButtonAnimationOffset);
   }];
   [animator addCompletion:^(UIViewAnimatingPosition finalPosition) {
     [self resetClearButton];
@@ -969,7 +969,9 @@ NSString* const kOmniboxFadeAnimationKey = @"OmniboxFadeAnimation";
 }
 
 // Accesses the clear button view when it's available; correctly resolves RTL.
-- (UIView*)clearButton {
+// This method must not be named -clearButton, because that conflicts with an
+// internal UITextField method.
+- (UIView*)clearButtonView {
   if ([self isTextFieldLTR]) {
     return self.rightView;
   } else {