Adding CRWWebViewContentView as a subview when it is set

Adding subviews in layoutSubviews is problematic
(see http://stackoverflow.com/questions/34301366/call-addsubview-within-layoutsubviews/34301753)

This CL moves it out of |layoutSubviews| and
into when the CRWWebViewContentView is actually
set on the ContainerView.

The existing logic will take care of removing
the old ContentView from the superview
when there is a need to replace the
ContentView.

As a (intended) side effect this CL fixes a bug in UIWebView
 where adding it as part of layoutSubViews would lead to issues
in scrolling of certain pages.

BUG=568097
TEST=
Illustrated in the bug.

Review URL: https://codereview.chromium.org/1531593002

Cr-Commit-Position: refs/heads/master@{#365691}

ios/web/web_state/ui/crw_web_controller_container_view.mm

@@ -160,6 +160,10 @@ - (void)setWebViewContentView:(CRWWebViewContentView*)webViewContentView {
   if (![_webViewContentView isEqual:webViewContentView]) {
     [_webViewContentView removeFromSuperview];
     _webViewContentView.reset([webViewContentView retain]);
+    if (_webViewContentView) {
+      DCHECK(![_webViewContentView superview]);
+      [self addSubview:_webViewContentView];
+    }
   }
 }
 
@@ -215,12 +219,10 @@ - (CGRect)visibleFrame {
 - (void)layoutSubviews {
   [super layoutSubviews];
 
-  // webViewcontentView layout.
-  if (self.webViewContentView) {
-    if (!self.webViewContentView.superview)
-      [self addSubview:self.webViewContentView];
-    self.webViewContentView.frame = self.bounds;
-  }
+  self.webViewContentView.frame = self.bounds;
+
+  // TODO(crbug.com/570114): Move adding of the following subviews to another
+  // place.
 
   // nativeController layout.
   if (self.nativeController) {