[iOS] Don't remove the old content view if re-displaying it.

BrowserContainerView's |-displayContentView:| should have no effect if
called twice with the same argument, but its current implementation
changes the view hierarchy in this case.

This CL is a partial solution to fix a specific issue with the voice
search button NamedGuide to minimize risk.

Bug: 826093
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ia2d58883df057df2ad430ecdbc7ccdf28f02a4cd
Reviewed-on: https://chromium-review.googlesource.com/981609
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546245}

ios/chrome/browser/ui/browser_container_view.mm

@@ -4,6 +4,8 @@
 
 #import "ios/chrome/browser/ui/browser_container_view.h"
 
+#import "ios/chrome/browser/ui/util/named_guide.h"
+
 #include "base/logging.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
@@ -21,13 +23,28 @@
 }
 
 - (void)displayContentView:(UIView*)contentView {
+  if (_contentView == contentView) {
+    // When voice search is launched from the NTP, the voice search button
+    // NamedGuide is constrained to a subview of |_contentView|.  If this
+    // occurs, removing |_contentView| from the hierarchy below will deactivate
+    // the constraints.  The check against the voice search button guide is a
+    // temporary workaround in order to minimize unintended side effects from
+    // this change; this function should be a no-op when |_contentView| ==
+    // |contentView|, regardless of the NamedGuide constraints.
+    // TODO(crbug.com/826093): Remove NamedGuide check and simply early return.
+    NamedGuide* voiceSearchGuide =
+        [NamedGuide guideWithName:kVoiceSearchButtonGuide view:self];
+    UIView* voiceSearchButton = voiceSearchGuide.constrainedView;
+    if ([voiceSearchButton isDescendantOfView:_contentView])
+      return;
+  }
+
   DCHECK(![_contentView superview] || [_contentView superview] == self);
   [_contentView removeFromSuperview];
   _contentView = contentView;
 
-  if (contentView) {
+  if (contentView)
     [self addSubview:contentView];
-  }
 }
 
 @end